I looked through his phone. What do I do now?

if you were sure it was just texts maybe you can forgive? but make him work for it! i had a boyfriend who was emailing women online cyber-... i was gutted cos it felt like he cheated on me, i forgave him as he didnt do nything physically.. and we were happy for a year. its entirely up to u but if u do tak him back tell him he cant text that girl again. purely out of respect for you as his girlfriend. good luck! xx