Cheating jerks

I just found out my SO is cheating on me after five years together. I am devastated and honestly feel lost. He left his phone at home and started to recieve texts from a person named Jason but Jason was sending some weird texts and when I called the number I found out it was a girl he had been seeing for three months now. I wasn't snooping but he kept getting texts so I wanted to tell the person that he didn't have his phone. Have you ever been cheated on? How did you find out?