Have you ever been cheated on/ cheated on someone?

I'm currently in a relationship that I'm about 90% sure that my boyfriend is cheating on me. I'm trying to deal with it responsibly, but I'm finding it to be difficult and I'm racking my brain over it. Whenever I imagine him with someone else (in a romantic sense) my stomach churns. Im just curious about how others have dealt with cheaters and, if you have cheated on someone, for what reasons did you cheat on your significant other?