What's a girl to do?

I have been with a man for 7 years. They were the longest years of my life! I had 2 children with this man - and he never worked, never helped with nothing - was never home when i was. I got fed up and left - put him out and continue to be the best mom that i can be. I find out he has another baby on the way - and that 2 years prior to me leaving his azz he was messing with her. He is 31, she is 23!!!!!!! Frustrated as ever. He does not even do for his 2 - how is he going to be irresponsible and make another! This fool keeps calling me saying he is sorry and wants this to work - and has been for the longest. It was him who told me of the "new" baby after the fact, and now he acts like he does not get why i won't mess with him. I know the next girl will feel the same lonely nights and the bull**** that i have endured. I know he is no good but why does it hurt so bad? why do i feel like i failed at this mess? He is now living with this "NEW" girl and when i think about it