Hello and welcome to the forum. It takes a lot of strength to tell stories such as yours and you have my heartfelt respect and admiration for doing so. Thank you for telling us your story.
Why are some people abusive towards others is what I hear you saying. The answer? Who knows. I know long term abuse and trauma can have that effect as the person concerned wants some revenge on the world but it is family members to who are on the receiving end and it seems a little unfair when all we have done is try to love and support that person.
I have been in a similar situation and in the end came to the conclusion that if I wanted to be my own person I needed to leave. It was not easy. It's not just the parting that hurts but the ongoing loneliness of being on your own. I know many women leave these situations but return because of this loneliness and their belief they cannot manage on their own. I'm sure you recognise this as the result of living in an abusive relationship. As you said, Why do some people do this to others, break them down.
In my opinion it is a power thing. The person must always be in control even in the smallest matters. And when they feel a loss of control they use all sorts of tactics to regain control and put the partner in their place. I am sorry you have experienced this and hope you can decide on a way out of the situation. Sophie has suggested you contact the 1800 RESPECT people who have much experience in these matters. They will be able to help you with your decision whatever it is. We can also help and support you on this forum if you wish.
Have you looked at the threads Sophie has suggested? Please do. And continue to write in here as often as you wish. We have all been hurt in some way so understand emotional pain no matter the reason for it. No judgments here.