A similar thing happened to me one evening when I couldnt sleep and found an email message from a girl 20 years his junior who my husband had an affair with for 9 months. More recently he'd been starting a new affair with a singer he met on the tube. There went our 20 year relationship, turned into trashy cliches.
I asked him to leave and over the next 3 months we went for counselling so I could work out what I wanted to do. Throughout the period he continued to minimise - or continue to lie as that is what it is - reinforcing the betrayal. He and her knew what had happened to my marriage but I didnt. In the end I decided it was over - he had broken my heart and was no longer the man I had thought he was when I fell in love with him. He was devastated - like he had sleep walked into destroying his life. As he said later to his sister 'I had the perfect life and I threw it away.'
Depending on what your partner does next I would suggest counselling. Not to keep you together but to help you to find out what happened, to hear each other and make decisions that have the least potential to continue to hurt you.
For me now, 8 years later we get on well, I bear him no ill will and can even pity the sadness for him of moving to his 'sad Dad flat' and away from living from his children - all brought about by his own weak ego.
At the time of course I was utterly devastated and even now I know I bear the scars. It was a deep dark hole of despair. So my deepest, deepest empathy for you at this time. But please know: you will survive. One day you will wake up and its not the first thing that hits you in the face. Right now, just get through each day at a time. Reach out to friends for support. Look after yourself. Find tiny simple pleasures when you can - in a little while start to plan a few things to give yourself something to look forward to - a weekend away, or a walk with a friend. Embrace the awfulness of your grief - for it is grief, but know by doing so one day you will have let it go. And choose living life - and the life you want to lead. It will feel for a long time that he's ruined your life but soon enough you will have the strength to fashion a new and different life for yourself if thats what you decide.
If you do decide to stay and forgive, do so for positive reasons. Because he can show you real contrition, love and respect. Don't stay out of fear - because over time that could break you into many tiny pieces. And fundamentally you deserve better. So so much better.