Maybe a conversation. You struggling to let him go doesn’t mean you have to call it quits you have a child together nothing is simple after the fact. You need to speak to him. Apologise to him of how you treated him and explain to him why you treated him that way and explain things from your angle not justify but explain to him you still want things to work that is if you do and try make things civil or better for the both of you. Things are hard having kids especially on mom and sometimes dads need to be reminded that we need support more support then most and no judgement
If your ex can’t understand and support you through postpartum depression you don’t want him. It’s never going to get any easier, if he didn’t take care of you while you took care of the human life you grew in your body, he never will. You gave him a child! A whole human. You did that for him. He can suck it up.