Hello Tadams
You have a way to go yet before you can stop trying to work out what went wrong in this abusive relationship but you are making a good start.
Your ex was probably only ever 'nice' to other people for his own reasons, to upset you, put on a false exterior to promote himself, gain something or even that he knew he couldn't make victims of some people. Men like him have to use a lot of charm to worm their way into peoples lives and affections which takes time.
As you probably found out, there is no point asking why he did these things, an honest answer is not on offer and more likely just because he could.
Have you asked yourself why you were the one to apologise? He was the one causing the abuse.
You have no need to prove yourself to anyone let alone him. You are who you are and if he couldn't accept that then that's his problem. In a good relationship partners value and respect the person they are with and do not want them to change. People like him pick on someone they can manipulate, anybody strong is too much of a challenge and they will lose the battle. You sound like a gentle caring person and maybe a little naive and the one in the relationship that puts in all the effort, in other words a suitable victim. He needed to be king of his domain and you gave him the opportunity by not rebelling. People like him are cruel because it's all they have got to use as a weapon to keep you confused and in your place.
I shouldn't assume that he was nice to his last girlfriend, no-one knows what really goes on in other relationships. She may have had her own reasons for staying with him for so long or she may have tried to put things right like you tried to do rather than give up and feel the collapse of the relationship was her fault.
You will get him out of your head when you see him for what he really is and see that he has the personality problem not you. He had a choice to treat you with respect or badly so I hope you can step back and see you are worth better than the crumbs he threw you. You will get your dignity back when you can see the facts and stop taking the blame. The only thing you did was try too hard and I hope if you do meet someone else they will cherish you for who you are.
Try to fill your life with things you like and people you want to be with because they care about you.
I wish you well and will follow your journey to get back to the person you have ignored for too long.
Regards
Jan