How should I handle the feelings I have for my ex S/O when entering a new relationship?

I’m in a complicated situation where I feel like I’m ready to move on from my ex (who is also the father of my 3yr old). We were together for a long time, bought a house and had a child, then all of a sudden he decided to end the relationship to be another woman, and things sort of went south from there. Eventually we reconciled, and went back and forth for a while, but later decided we did not want to be together. A lot of that had to do with my inability to forgive him. I also tried to start a new relationship once, but it eventually came to an end because i still felt guilty for being with someone else besides him, even though i did not really want to be with him after all that has happened. What should I do to get over that guilty feeling? and i guess more importantly why do I feel guilty? Any ideas?