Can I be mad? What should I do?

Sarah • 19. Rilee Page ❤️ 8Ibs 1oz, 21 inches long 😍 5.25.15
My SO went to hangout with a friend today and he left his phone behind. Me being nosey I went through it &I found that he was messaging girls on Facebook when he was telling me we were working things out. Also we are together and have been since we had our daughter a month ago and I saw him on this app whisper wanting to sext other girls and was telling one that I don't make him happy and he only stays cause he's comfortable. Can I even bring this up because I shouldn't have gone through his phone, but obviously it's a good thing I went with my gut and I did. I just don't know what to do/: what's your opinion ladies