I did, I found messages and pictures from two women on my partners phone. We split up, but reconciled a short while later.
We were living together at the time, I was absolutely heartbroken as our relationship was otherwise perfect. For a few months after our reconciliation, he slept on the sofa and we took small baby steps forward.
During this time, he actually did it again(!) and kissed a woman at a bar.
After that, I was adamant I was done, I felt embarrassed and stupid, yet I did still take him back. We repeated the above- living separately but together, only this time I could absolutely tell he was trying to fix his mistakes and work on himself. That was a few years ago, and we have been happy since.
It's not an easy road, what was very important for me was his patience when I took five steps back (I would become so upset I was sick on an almost weekly basis). The trust is somewhat back, I have since checked his phone a few times and found nothing- whilst I know I shouldn't have to and that some may think this is wrong, finding nothing has helped me build the trust back. He no longer drinks as he found (of his own accord) that he can't tell when to stop and becomes too drunk (example: him kissing the woman in the bar).
If it wasn't for him making the positive steps to find what made him do this, and create change, we wouldn't have made it.