Steven's poor mental health was hinted at in episodes like Mindful Education where he was having visions of Bismuth, Jasper, and Eyeball. And in Rocknaldo when it was shown that he was having trouble sleeping. They didn't show it upfront, but rather implied it.
That is true - I forgot about those. I feel like they hint at a different problem, though, rather than Steven's breakdown in SUF - like he is feeling grief for not being able to help people and instead had to 'destroy' them, rather than a deepseated anger and resentment. Might just be me tho 😎 But yeah - defo what you said. His identity crisis (who is he? What is he for?) was also painted clearly a bunch of times too.