I'm not too sure that you should be 'coping' with this. If you're not the only one you need to find a dude that will make you the only one. If you mean how do you get over the fact that your partner has cheated on you, then he was a douche in the first place and you really shouldn't care; we meet bad people sometimes, so it happens. Find someone who respects you and don't settle for disrespect. I do think you're worth more than that.
Cope? Like find a way to get past the idea the other is unfaithful and keep the relationship going? YOU DON'T!
Don't settle for less than you deserve. confront him or her and let them know you KNOW & how hurt you are by things. The next part "leaving" is up to you. You CAN stay but I would be sure that I became the ONLY one or set some rules in place.
But hm... what if this is about the only ONE on planet earth, Omg when did you discover we're not aloneeee. AAckskjs.