Crolwey did not know that he was in love with Aziraphale until Nina mistook them for a couple. Crolwey paused after she walked off and mutterd "love?" to himself because he wasn't aware that was what that feeling was. He knew he wanted to spend the rest of his days with the Angel, knew he wanted to drink wine and make jokes, knew he wanted to protect him at all costs, but he didn't know that was love. At least he didn't know all of those emotions were romantic love. So when he figures it out- at the most awkward time (while they are trying to stop Heaven and Hell coming for them)- Crowley doesn't say anything. Mainly because he isn't sure Aziraphale knows what those feelings are either, and also because the conversation goes back to 'Jim', a.k.a Gabriel, and so Crowley has a newfound anger. He doesn't kill people, doesn't kill anything, and yet he tells Gabriel to jump out of the window. Crowley has realised his feelings and realized that the one person he loved was told to 'Shut your stupid mouth and die already', and that's not sitting well with him. Why would it? He hates Heaven with a passion but doesn't hesitate to go up there to save Aziraphale. After expreciancing all of this in just a few hours, the devastation of Aziraphale rejecting him (another post about that whole thing coming soon) hits like a brick wall. He has only just realized what he had nearly lost, and luckily had gotten back (in season on, and now this time, he was losing Aziraphale with these feelings right in the front of his mind.
Aziraphale though, he has known since 1941, when Crowley saved him from the Nazi's in the church. He can recognize love. We know that from Season One, but we also know he reads. Alot. He loves books. And he has more than likely read romance novels, and we know that he (at least I assume) talks to his neighbouring shopkeepers, so Aziraphale probably understands emotions better than Crowley. And defiantly understands Love more than he does. So Aziraphale isn't shocked by Crowley's 'confession'. He is slightly taken aback by the way Crowley is so clearly emotional over it after never being before, and never denies that he has feelings for him too. Aziraphale assumed that they were both aware of those feelings but neither of them said anything or acted upon it because of Heaven and Hell. But now they don't have to worry about it, Aziraphale is much more open to these things. For example, walking about with Crolwey in public and not seeming to rush back to the bookshop. Or him arguing the Bentley is THEIR car, in public. Or him inviting Crowley to dance (again, another post about this stuff coming because I'm obsessed). Aziraphale assumes Crowley just understands it because he does, like they each assume they know what's best for each other on multiple occasions.
Crowley realizes hours before Aziraphale leaves him, and Aziraphale has known for close to 100 years.
Which really makes the ending sting so much worse.