It does hurt to have someone you care/cared for "choose" to be with someone else no matter how right you are in knowing he isnt good for you and you should not be with him anyway. It is just human nature. Just keep telling yourself that you are better off that he didnt "choose" to be with you. One day it will no longer matter.
I think your mom is right on target. They are both damaged. Just keep thanking the heavens you are not involved in that mess.
He is sick. I am not a man, but no you dont ask your soon to be ex-wife for sex while waiting to marry someone else unless you have serious issues. Even though he has left you and found his "true souldmate" (pardon me while I throw up) she isnt making him happy either or he wouldnt be coming on to you. She isnt the answer for him either, he just cant admit that now. He needs serious help and it needs to be from a professional. It is just too bad that somehow (not from you!!!) she couldnt find out what is going on behind her back. But just you wait, she will find out once the ink is dry on that marriage cert and he starts to feel like that certificate gives him license to do whatever he wants to do. Right now he has to look like the perfect family man or he will miss the next gravy train.....Not your problem anymore and the hurt will lessen for you just give it some more time......