How do I get over this heartache?

I was with my ex fiancee for 3 years. We lived together for almost 3 years and I have two children for someone else. We have been through alot of ups and downs, he was abusive sometimes, cheated, lied but on the other side of him he was wonderful. Bring me flowers just because, take me out to eat and movies, always thoughtful and considerate. He was the worst and the best man I have ever been with. I finally left him because the bad was outweighing the good. We still talked to each other, slept with each other and did things together since the break up 2 months ago. However, I just found out that he is "messing" with another girl from his work. She is married with kids - but he is supposedly "in love" after a week. This is the most painful experience I have had to endure. I don't know where to turn. i can't eat, i can't sleep, all i do is cry and think about him touching her and talking to her. I don't want to feel this way anymore, but I don't know how to move on with my life and let him go. Please help.