Closure isn't something the other person gives you, it's you accepting things as they are. You didn't do this during the relationship. You may have accepted the things he was doing, but you didn't accept the reality that that meant he wasn't a viable partner to you.
Your anxiety comes from not listening to your own feelings in this way; you knew he was hurting you, but you stayed, knowing you'd be hurt more. Who wouldn't feel anxious, knowing that the one person who's supposed to look after us is knowingly putting us in the path of harm? YOU are the person who is supposed to look after you. YOU were the person letting you down.
This might sound harsh, but what it means is that, although he was at fault in his behaviour, YOU are responsible for your own wellbeing, and as soon as you start to behave responsibly with it, you will feel less anxious.
So, would a person who has your back say 'You stupidly stayed with him', or 'No wonder your mental health was bad, you opted for a relationship with him', or do you think they might take a more supportive tone? When you can find a way to deal with yourself in a more supportive tone, you'll be able to accept that his lack of giving you closure is closure in itself. There's nothing more you need to know about him. You'll never have answers with regard to his motives and behaviours, but lots can be gained by figuring out yours.