yuck. It hurts, I know. I had a similar experience about six months ago, I was with my bf for five years and found him in bed with his new girlfriend(they had been dating for months under my nose).
I felt horrible, heart shredded, the whole bit. But I feel so so much better now.
Truth is, our relationship had not been as good as it once was, like yours, and now I am free to find someone who will respect and love me...and be honest. It feels great.
So take heart in knowing that in a few months, it won't feel quite so awful, as long as you refrain from any contact, which is tough.
I did it by marking off the days on a calendar with a happy face, one smile for every day I didn't text or call or see him.
Childish, I know, but it worked.
It gets better, I promise.
You didn't deserve it, but next time, you'll meet someone worthy.