Goh displays a lot of symptoms of low self-esteem. It probably started at a young age, both from his parents neglect and his difficulty with making friends. He puts on a confident front, but for anyone paying attention it's easy to see through it, especially when he blushes so easily.
As a way to protect himself from feeling worthless, he tends to give up easily and avoid doing anything he doesn't feel good enough at. He even says, "If you can't do something, give it up, right?" And he doesn't start to change that way of thinking until he sees how it's affecting his relationship with Scorbunny.
He doesn't speak up when he's upset or share how he’s feeling, and considers others to be more important than himself. Like when he prioritizes his parents happiness over his own, and tries to act like he's doing fine when he's not. The same thing happens with Raboot too, Goh thinks he's making it unhappy so he tries to leave it even though it makes him unhappy to do that.
When Ash notices the signs that Goh is upset, he starts to ask Goh when he thinks something is wrong and it gives Goh more opportunities to express how he's feeling.
Goh feels like he has to be correct all the time to prove himself, which can be tiring for him to keep up with. But he relies on his knowledge to show his worth and to get praise and attention.
He also gets really upset and stubborn when people don’t agree with him, resisting their viewpoint until something big enough happens to change his mind. Like how he's very antagonistic towards Gary, until he starts to realize and accept that Gary's criticism can help him improve.
And Goh is also very self-critical. He's a perfectionist that sets high standards that he feels he has to meet, otherwise he's not good enough. Mew has to be his first Pokemon, he has to do everything on his own, and he has to excel at everything he does. Even towards the end of the series, he's still struggling to balance his lofty goals with realistic expectations for himself. During the Project Mew arc, he felt like he should have been able to handle the situation without relying on his Pokemon.
Part of the reason Ash has helped him so much is because of all the encouragement and constant reminders that he's there to support him. Goh wasn’t really used to being praised like that before, so it helped him start developing a better mindset. And just having a close friend, that actually stuck around and pays attention to his feelings, probably helped boost his self-esteem too.
Goh is still working through his self-esteem issues at the end of the series. He even fell back into some of his old habits after Ash said he was leaving on a journey, but he has made a huge amount of progress and has lots of friends to support him now.
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