So, let me get this straight?
You have expressed your concerns to your partner that having a relationship with an ex is a boundary that you'd consider crossed, and he instead told you that you're trying to control who he is friends with, instead of taking your concerns on board like a respectful partner?
And because of that, you think that the problem lays with you? Oh honey, no.
Of course you can't control who he is friends with but any partner worth their salt would respect the boundaries of the person they'd potentially be spending the rest of their life with. He's saying his relationship with his ex is more valuable than your feelings.
And also, he doesn't get to just be "useless". He doesn't get to play Strategic Incompetence so that then you'll just do it because he's no good. He has parental responsibilities too.
I'm not surprised antidepressants haven't helped, because it's not your brain chemistry that needs changing. It's the situation you're in that needs changing. You're going to be living life constantly anxious and treading on eggshells around this man who is making you feel like you've got problems when he's being blatantly and utterly disrespectful.
Is this a relationship you honestly wish to pursue? And do you think he's pursuing the relationship with you for all of the right reasons?