He's a liar and a cheat. And he played on your (understandable) sympathies with that cock and bull story about his mother - who I suspect had no idea he was using her as an excuse to go get laid.
Step back and assess this. What would you tell a friend?
You'd probably tell him or her to cut loose as soon as possible, and that hanging around would only make it harder to let go later. You would probably tell your friend that the person who cheated on them was a proven liar who was probably also lying about commitments and marriage and kids, etc. You'd probably shore up your friend and tell him or her that they are attractive and worthwhile and that they deserve better, and that someone will come along and will make them forget all about the lying cheater.
Now look in the mirror and tell yourself that, because you deserve to treat yourself as well as you would treat a friend.