OP - The way you have to deal with it is just accept the fact that your EX is a deceiving person. You don't know if your entire relationship with her was just a lie or was the truth.
She's cheating now, she could have been cheating with others prior, you have no way of knowing. Once a cheater, always a cheater.
You are mourning the death of a relationship YOU THOUGHT you had. In reality you were deceived, you were played and you were taken advantage of. Once you accept this, you will quit thinking of her, you won't want anything to do with her and you will move on.
Here's another probability. When her AP dumps her, she will come crawling back to you. This almost ALWAYS happens when a cheater looses the "Lust" phase of her current relationship. You must prepare yourself NOT TO TAKE HER BACK when this happens.
I've learned a long time ago, a woman can not love a man she doesn't respect. She has been showing you the ultimate disrespect and if you take her back, she'll respect you even worst. If and when she comes back to you, its because she wants your resources until she can find your replacement again.
You must become a Alpha Male, Beta's get walked over and used. Hence why these women seem to always flock to the "bad boys" and use the "nice guys". You need to channel your own "bad boy". Remember any red flags that are seen, be willing to walk. If you walk, you never take them back. They need to know that if they cross you, they lose you forever.
I know it sucks, but we all learn this lesson the hard way at one time in our lives.
Right now, it seems like your breakup is recent. Just focus on yourself, go to the gym, re-associate with old friends and do things you have always wanted to do. Keep busy and completely go no contact with your ex. No exceptions.
[This message edited by Txquail at 10:09 PM, Sunday, February 12th]