What do I do??

So I had been having nightmares that my bf was cheating on me for weeks. Finally yesterday when he was napping I went through his phone to get the weight off my shoulders. I found him dming a girl and consoling her because her bf cheated and lied to her. His response was “I’m sorry but if you were mine I’d treat you right”. I woke him up and asked what it was he said “that’s old” I said clearly it was from last week. He then apologized profusely. I’m just so undecided I’m sickened at the thought of him even doing that because he’s always been so amazingly loyal. I’ve felt nauseous and constant aches in my body and I can’t sleep I don’t know what to do. Like is it bad that I’m not 100% sure I wanna break with him?