First ever post! Spent a lot of time reading excellent advice from this forum in the past and it helped me define boundaries in the past when I really needed them - thanks for that.
I've been separated from my ExH for nearly 4 years, we have one DS (8) together. He has a new partner and 2 additional children with her as well as her DS (9).
He was a real arse, self-confessed lack of empathy or understanding of people's feelings, fits of rage, constantly berated me for not being good enough at anything, essentially being an inadequate person.This is really the tip of the iceberg - there were times he pressured and pretty much forced me into sex, the time he pinned me on the bed and shouted in my face, friends he insisted I never see again, relationships ruined forever... the time he threatened to beat (then baby) DS because he wouldn't go to sleep... the list is endless.
Doubtlessly he was an total twat and I did the right thing in leaving... I've never felt the level of stress and emotional break down at any other point, I'm not sure how I survived it. He was in a furious state of disbelief when I left him for a long time.
I built a new life with my DS, we now have a great life and relationship with each other and all has finally stabilised. We have shared care which is OK, DS feels he is loved by all involved, loves his new siblings and feels we all get on..
The issue I now have is - how oh how oh how is his new relationship so successful? Does this mean it was me in some way? Why is she OK where I wasn't?
I feel so lost and confused about how to make a connection again with someone new. How do I put this behind me enough to seriously consider someone new in a relationship? I've dated plenty of people since then, one for 8 months who was lovely, but I don't seem to be able to consider someone as an actual partner. Suspect I may now have the dreaded commitment issues 
Sorry about the length of the post - thanks for reading, any help much appreciated...