Help! Why Want And Ex Back W/ Plenty Of Issues?

So This is my story. I was in a relationship with he father of my 2 kids for 6 years. He cheated on me for 5 out of the 6 years. I stayed with him for so long for what I thought was "love", after our last year together he had a child for a women he was sup-post to have been with for 4 out of our 6 years. They are now together and live together with there child. this girl is 4 years younger then me. I am 22 he is 22 and she is 18. I am not doing bad career wise and I just bought a new house. But for some reason I still want him back in ways? I don't know if its out of spite or just to prove the fact that I could have him if i wanted him. But now I want him back? WHY?