How can I deal with praying for my enemies?

I feel wronged and hurt by this man and now his girlfriend has joined in to both mistreat me. We were never in a relationship, but he played me like a lot of other women into thinking they were special when he just used women selfishly. The new girlfriend is overprotective of him so she acts jealous towards me. I've been nice and even smiled at the girl. She still doesn't like me. The guy rubs in the fact he's in love with her and "forgets" that he acted unkindly. I haven't retaliated or told him just how much it stinks how he acted. I'm having a hard time moving on from dwelling about the hurt. Why is it so hard to forgive sometimes? I feel like I want justice done. It makes me feel worse to see that they are in love and he has left hurt women behind him. I try to stand out of the way from being hurt by them, but how does forgiveness fit in? When you see people seemingly better off, why pray for them? Or should I pray for the mother of the guy's little children who's probably suffering more than I am from his behavior?