My only advice is to realize how lucky you are that you didn't get married to him. Additionally, consider it a blessing that you found out about it when you did, let alone having found out at all. Consider it a blessing that you don't have to waste any more of your time with him. The girl he's with now realizes what a piece of trash he is, so at least there isn't much that he can conceal from her (at least until she finds herself in your position).

Consider yourself blessed to not be where she is now. The best form of revenge is living a happy and fulfilling life of your own. How can you get over him? By knowing that you're better than he is or ever will be. Don't worry about getting that apology from them. Hold your head up high, be proud of who you are, and take every opportunity to show off your wonderful new partner in front of them.