Now that I have somewhere to yap about this stuff...
Cosmos deserves more respect than he gets, both in the fandom and in the show.
Before i say anything, I am in no way or form excusing certain actions of his, this is just my personal opinion.
Cosmo's trauma has never been properly addressed throughout sams, it's always either pushed away in favor of a different character, or just treated like a joke. His death is literally a joke to his own family and a lot of his friends, if I was him, I'd feel like nobody actually cared for me out my wellbeing. He's never tried to push for for respect or boundaries either, because his coping mechanism is just hiding it and pushing it to the back of his head.
Terra's therapy obviously didn't help, even when he finally got more comfortable with talking to her about his trauma, he found out that she became friends with the main source of said trauma while being fully aware of it, from his perspective it seemed like she didn't actually care about him.
If she didn't care about him, why should he tell her anything? It made him go back to the mindset of needing to hide his issues and be better for his family.
And then the astrals came.
He was basically forced into a cult against his will, given powers he never wanted, and basically had his own identity ripped away from him. He says he's fine with how everything turned out for him, but I honestly doubt he really is.
Im not excusing him crippling Terra, but he's been bottling up trauma, and forced to basically stop feelings his emotions properly by the astrals.
He isn't okay, and considering that he is an astral, never got a true apology from anyone really, and still hasn't had of his trauma actually addressed, I doubt he ever will be.
Everyone in this family deserves better, and they clearly aren't getting it from each other.