If such a big change wasn't enough, we also have to process the headfuckery we have been put through, it's little wonder you're feeling as you do @RibenaToothkind . Have you had any support from Women's Aid or similar? They get it. They often have meet ups for women moving forward from their previous lives, it might be worth trying your local branch to see if they have anything like that going on just now.
which somehow makes me feel even worse, that I caused it all by not being strong enough
No. Absolutely not. The reason he treated you the way he did was because he isn't strong enough. Strong people don't bully or abuse. It's their insecurity that has them wanting you to be 'lower' than them.
I should have just stood up to him more and I'm honestly not sure why I didn't
I look back at some of the events I went through with my ex and wonder why I reacted, or didn't, the way I did. It would be very different now, because I have had a few years away from him and I have become myself again - regained the strength that was always in there. We are trained from day one not to stand up to them. From day one.
As I said, fortunate are those who don't understand. Those who just spout 'advice' to make themselves feel better for having offered it and in so doing choose not to listen to you, it's not unreasonable to distance yourself if you feel worse after having spent time with them.
Over explaining yourself is part of the abuse. One of my regrets is how much I shared with him, right from the start, but they make you trust them; you think you can share your deepest and darkest, which sets you up nicely for the "but, but, buts ..." you'll be doing for the following however many years.
It is not your fault @RibenaToothkind , it never was. If it was yours it was mine too and all the other people who have been and are going through the same experiences. It is only ever the abusers who choose to abuse. They may have had a hard childhood, millions of us have but millions of us don't go on to abuse those we are supposed to love and cherish. He chooses to behave the way he does. He knows right from wrong. If he didn't your friends would have seen what you experienced.