I found messages on his phone and now I cant unsee them

We have been together for 4 years. Last Tuesday he left his phone on the kitchen counter unlocked. I was not snooping, I just saw a notification pop up from a name I did not recognise. What I found when I opened it changed everything. He has been talking to someone from his work for 8 months. Not just talking. I have not said anything to him yet. I go to work, come home, cook, sleep next to him like everything is normal. I do not know how much longer I can do this. Has anyone been here before? How did you handle it?