You cannot make the pain go away, unfortunately you just have to feel it, along with the heart break, the fear, the loneliness, the disbelief.
You want to turn back time, to unknow what you know. But you can't.
Whatever you decide in terms of whether to stay with him or not, you are going to feel pain. You have been cheated on by the person you trusted above all others. The person you chose to trust. It's different to how you love and trust your family (if you do) because this is someone you chose to have your back, to put you first, to support you, to be there for better or worse.
If you stay with him you will still feel devastated, you'll feel insecure, worried every time he texts someone or goes on a night out. He will under mine your feelings by telling you that you need to get over it, to move on, to stop talking about it because you're upsetting him.
If you go you will feel fear and uncertainty, the rug has been pulled out from under the foundations of your life. But you will emerge a stronger person for it.
No one can judge you for what you decide, you are the only person who walks in your shoes. You know yourself, your life as a couple, your own personal hopes and dreams. But try to think objectively, what would you tell a friend or lived one to do in the same situation.
I can understand your anger at the OW, it's easier to feel angry at the one that you're not emotionally vested in. Your anger towards him will come in time, along with a whole other range of emotions. You're going through the grieving process, grieving for the life you had the day before you over heard him, grieving for the future you thought you would have. But whatever you choose to do that grieving process will take place, you can't stop it. It's hard, it is really fucking hard 
