I do understand you. I was with someone from 14 to 18, he was a couple of years older, and we had a fairly volatile relationship, arguing and splitting up, getting back together, for the last year or so of it.. and it culminated in him hitting me. I was 5ft, he was 6ft4... absolutely no way I could possibly fight back really.. I tried to tell myself he was genuinely sorry when he was apologising, but I couldnt take away the fear after that, that every time we argued he would hit me, and the trust was gone. I ended the relationship.
We are in touch again now, 25 years later, we chat on FB occasionally, and he has been happily married to the next person he began a relationship with after me, for all this time.. and he has never hit her. Me and him wouldnt have worked.. maybe he did learn from that incident, but I could never have carried on the relationship always wondering if he would do it again.
In my mid twenties I met someone, lived with him, and subsequently found out that his previous relationship had involved a lot of violence, on both sides.. sadly that was part of his character and he hit me. I owned the house we lived in, and it was quite possibly the worst time of my life as the violence just escalated, and I was unable to escape him easily. But I did get away.. I invented a tale about how I was being sent on a course for two weeks.. packed my bags and drove off... I had already made plans to stay with a friend in London, had arranged a company transfer to London, and had sneaked out my photos/valuables/passport etc and any other important small item I needed. I didnt give a toss about anything else in the house. I let it get repossessed. He had threatened if I made him move out, he would have burnt it down with me in it, and I have no doubts he would have tried.. he was an utter psycho. I was shaking like a leaf the day I rang him from my mates back garden in London and told him that I wasnt coming back.. but the relief was amazing.
I heard later from friends that he soon had a new woman, and was beating her up... so no, he was never going to change.
Sorry about all that long post, but I just wanted to point out that I have experience of both... someone who did stop.. and someone who didnt, but for me, one punch would be enough for me to end it.