You couldn't have been more battered by this man if he had beat you with a club.
You have asked me to help you understand why he behaved the way he did and what he gets from it. There are several possible causes of his behavior. First, and one that is surely true, is that he hates women. As you can see, he treats women like objects to use and throw away. His hatred for women likely stems back to his relationship with his mother. He probably hated her and now takes out his rage on all women.
Another possible cause of his behavior is a fear of intimacy. By sleeping with a lot of different women, he can avoid becoming too attached to any one woman. He may also be afraid of rejection. By cheating on his partner the way he did, he arranges to do the rejecting before it can be done to him. Yet another possibility is that he is homosexual and doesn't know it. By sleeping with many women, he may be unconsciously trying to prove to himself that he isn't gay.
Last, but not least, the fact that he chose to sleep with a relative of yours makes me wonder if he was incested himself. Victims of incest will often recreate their trauma by incesting others. By sleeping with a relative of yours, he engaged in a type of incest.
Now we need to focus on you. You said that you tend to be soft and that you are afraid that you won't have the strength to keep him away from you. Your fear is telling you something very important about yourself: that you are drawn to men who mistreat you. The only reason that you would be drawn to this type of man, and take him back, is because you, yourself, were abused in childhood.
As I have said again and again, we all recreate the wounds of childhood hoping to heal them. The first step in the recreation process is to choose a partner who will abuse us the way our parent(s) did. No matter how much the parter abuses us we can't let go (or we take them back) because we desperately want to achieve a resolution to the childhood trauma. The fantasy is that this time, if I try really hard, or if I am really good and forgiving, I will be rewarded with love and respect (or whatever else you felt deprived of by your parents). The urge to keep trying again and again reaches compulsive proportions, hence the reason why this pattern is called the repetition compulsion.
See my Advice Archives under repetition compulsion and unfinished business for more information. You will need to keep reminding yourself that a leopard never changes its spots, once an abuser always an abuser. You are never going to get your 'Happy Ending' with this man. Reminding yourself of this will help you to resist the temptation to take him back.
Hang in there and take care.