Since Chihaya was never good at putting her feelings into words, her thoughts during her confession were sort of left as a small puzzle, for the readers to solve on their own.
I feel that we have to break it out panel-by-panel, to better understand her in that scene. We will start after Chihaya's conversation with Desk-kun, where we have these 2 panels of Chihaya (these 2 pages were added in the volume):
As far as I'm aware, there were only 2 times in the manga, when the author drew Chihaya in that "blurry" way, where we see 2-3 "layers of Chihaya" overlapping with one another, indicating a shocking realization. The second one was in the last chapter, and the first one was when Taichi confessed to her:
In chapter 138, Taichi's confession finally connected the dots for Chihaya, and gave her the answers to those questions she was asking herself: "Why does Taichi always seem so stressed?", "Why does he look like he's suffering so much?", "Why is he always alone?"...Because he was in love with her. In chapter 247, Chihaya finally connected the dots, and gotten an answer to why Taichi was always in her heart. Why was she so hurt when Taichi seemed to be suffering, and why she wanted Taichi to laugh (ch135), why she felt his presence in the nationals (ch154), why she imagined him at the Omi Jingu temple before the queen match (ch216)...Because she was in love with him. Suetsugu drew both realizations in the same style.
Chihaya then goes to find Taichi, and they first talk about the picture they all took after Chihaya became the queen. Chihaya tried to cover for Hanano by saying that it was her stomach that growled, however Taichi was aware that she was covering for her:
In the 3rd panel above, we see Chihaya being surprised that Taichi knew that it was Hanano's stomach that growled. She proceeds to gaze at Taichi in awe, as she realizes that:
1. Taichi's hearing improved so much that he's now able to even distinguish Chihaya's stomach growling from other people (even though he was the farthest from them when the picture was taken). 2. By now, he knows Chihaya so well, that he can even recognize the sound of her stomach growling from all the time they've played karuta together.
It was kind of an indication that it's pointless at this stage for the two of them to lie to each other about anything, even little things, because they know each other too well for the lie to work. Which leads us to the next page:
Chihaya starts off with "Ta............", slightly blushing with a shaded face... but cuts herself short, and asks Taichi about which university he was going. Chihaya was about to tell him how she felt right then and there, but changed her mind at the last moment. And the reason is simple - She wasn't sure whether Taichi was still in love with her, since she overheard what Taichi said to Kana in ch207 about how his feelings for Chihaya may be fading away, and it's been about 3 months since then. Furthermore, Taichi leaving Tokyo (and thus leaving her) was an indication to Chihaya that he may no longer feel the same way about her, which is why she cut herself short of confessing to him.
If she were to confess to him, while Taichi's feelings weren't the same anymore, it would only put a burden on him. Just like in ch142, when she didn't want Taichi to find out that she'd also quit the karuta club, because she thought it would put a burden on him.
However, it is the panels at the bottom of that page that gave Chihaya the answer, on whether Taichi still holds those feelings for her.
4th panel: When Taichi said that he felt like he had to change his surroundings, he couldn't bring himself to look Chihaya in the eyes. Whenever Taichi is hiding something, he can't look the other person in the eyes. This has happened several times in the manga by now. For example, in ch134, Taichi couldn't bring himself to look Arata in the eyes when they were about to play for the first time, because the real reason why he stayed behind to play against Arata, wasn't something he could tell him. And in ch247, the real reason why he was leaving Tokyo, wasn't something he could tell Chihaya, so he couldn't bring himself to look her in the eyes. 5th panel: Chihaya sees that Taichi's body language holds more to it than what he said. As I mentioned, the two of them know each other way too good to effectively hide things/emotions from one another. She sees that Taichi seems stressed, and that he's trying to hide his pain.
6th panel: This is her realization - “Oh, it's because of me.” A year ago, when she was asking herself why Taichi looked stressed, and is suffering, she was thinking “it's because of karuta”. But now she's able to put two and two together. She realizes that Taichi is still in pain because of her, that he's still in love with her, that he's still suffering when he's close to her, and that the real reason why he felt that he "had to change his surroundings" was because of her.
Most likely, what solidified Taichi's decision to leave Tokyo, was when he saw Chihaya and Arata together after becoming queen/master, and couldn't hide his pain seeing them together on those stairs:
His feelings never faded away of course. Back in ch207, Taichi knew that Chihaya would overhear whatever he was gonna say, so he chose an answer that wouldn't put a burden on Chihaya. But, I'll leave that scene to dissect panel-by-panel in some other post.
Back to the topic, Taichi tries to hide his pain from Chihaya the best he can, but ineffectively.
When Taichi saw her dumbfounded reaction, he tried to put even more effort by forcing a smile while saying to her the same words Chihaya said in ch6 when whey were little. How do we know it's a forced smile? Because it's the same smile as in ch138, which happened just 2 pages after Chihaya's rejection:
Suetsugu improved her art drastically since then, but nevertheless, it's evident that it was a forced smile. Maybe not as forced and painful as the one from ch138, but forced nevertheless:
Taichi was trying his best to make Chihaya feel better about him leaving, and Chihaya could see that. Again, Taichi couldn't bring himself to look Chihaya in the eyes, and in the 3rd panel, we can see him diverting eye-contact completely, by looking down, trying to hide what his eyes would tell her.
Chihaya observes him, as she realizes what he truly feels from his body language alone. How can we be sure that that's the case? Because Tsuetsugu already drew Taichi in this way before. All we need to do is compare the panel of Taichi above, where Chihaya is silently looking at him, from the scene in ch135, when Chihaya crumbled over Taichi's suffering. It's the same body language. The answer couldn't be more clear:
Chihaya realizes that Taichi is still in pain because of her, his pain never stopped, ever since his confession from last spring, which is where her mind goes next:
As she remembers Taichi's confession, she ends her thoughts by thinking that it was a poem. To Chihaya, Taichi's love for her, was a poem. All those years of fun and joy that happened to her, was mainly because of Taichi. From starting the Mizusawa club, to forming a team, to going to the nationals, to winning the nationals, the friendships that she made on this journey, the love for karuta that grew on Chihaya that led her to becoming a queen... None of it would have happened if Taichi wasn't by her side. None of it would have happened if Taichi didn't love her. And the thought of losing this, losing Taichi's love, is what finally made her collapse.
In order to better understand Chihaya collapsing, we need to take a break here, and go back to the beginning of the story. In fact, we already had two times, when Chihaya almost collapsed because of Taichi leaving her. First, in chapter 4, Chihaya started wobbling, and hit the desk, after finding out that Taichi is going to a far-away middle school, so he wouldn't be able to play karuta with her anymore:
Chihaya actually did collapse in chapter 5, but back then, the impression was that she collapsed because both Taichi and Arata were leaving her:
Afterwards, Taichi and Arata argued over whose fault it was that Chihaya quit karuta, and the story didn't give us a concrete answer at the time:
However, the story eventually gave us many hints to piece the pieces together, and help us make a conclusion that Arata leaving to Fukui, was simply adding salt to the wound that was created by Taichi. But, it would take too long to analyze that conclusion in detail.
Instead, we will focus on something more simple, and easier to understand - Chihaya's forced smile:
In chapter 4, when she gave us this smile, as she was worried over Taichi quitting karuta, we didn't understand Chihaya well enough to fully comprehend the emotions behind such an innocent smile. But what's interesting is that we get almost the exact same forced smile from her, including the finger gesture, in ch135, right before Chihaya started crying over Taichi:
We get Chihaya's forced smile in ch138 as well, right after Chihaya found out that Taichi quit karuta, and right before she broke during her speech. And we also get it in ch184, right after Sudo agreed to Taichi's “loser quits karuta” bet, indicating that she was on the verge of a mental breakdown over Taichi, right before her match with Rion started (which is why she couldn't play her best in that match, and only after Taichi had won against Sudo, Chihaya saw the cards about to be read - floating):
To wrap it all up, we finally get back to ch247, to the scene when Chihaya found out that Taichi wasn't staying in Tokyo... It was such a big shock to her, that she started wobbling just like in ch4 (noticed the same “wobbling effect”), and Michiru, who noticed it, grabbed her arm so she wouldn't fall.
When she finally collapses on the tatami mats in front of Taichi, there was no table, and no Michiru to stop her from falling. As we had observed, Chihaya collapsing over Taichi was a reoccurring thing since the beginning of the manga. And Chihaya always thought that she was hurt because “Taichi was leaving karuta”, because ever since she fell in love with karuta, she was seeing the world through “karuta goggles”.
But Taichi wasn't leaving karuta in ch247. He even said that he'll be back to challenge Arata for the Meijin title. And yet, Chihaya collapsed. Because, in reality, her mental breakdowns and worries over “Taichi leaving karuta”, were all this time really a curtain for “Taichi leaving her”, which is something that she only realized in the final chapter.
And since Chihaya is too much of a karuta-idiot to put her feelings for Taichi into complex words, what spoke more than a 1000 words during her confession, was her body language:
The way that she gently held Taichi's hand with both of hers, as her body language shows repentance for hurting Taichi so much, and complete and utter gratitude for everything that he did for her, for helping her dreams come true, for being by her side, for loving her in the past, and for still loving her in the present.
Just prior to this, she was thinking how as soon as she hears the first syllable, she can recognize the card, but she can't compose the rest of the poem from it. She proceeded to think how Taichi's confession to her was a poem, but that she isn't capable of composing one. So she simply says to Taichi "I love you", or in Japanese "Suki da yo". But the great part in that scene is that 4 bubbles are used in Japanese, 1 bubble for every syllable: "su", "ki", "da", "yo". So she said her confession in syllables.
That was a throwback to chapter 49, when Chihaya was saying only the first 5 syllables of each card. Even though that sounded like mumbo-jumbo to others, to Kana, it sounded like a poetry, because of the way Chihaya was saying it:
So even though Chihaya was thinking that her confession was far from being a poem, it was definitely a poem to Taichi. The most beautiful poem he ever heard.