If you know your partner is cheating, then the best thing to do is talk to them directly and give them time. Ask them if they want to be in a relationship or not. Then give some time to think about it. We know it can be a painful situation for a loyal person, but being single is a thousand times better than being in a wrong relationship. It's better to wait for the right person instead of living with a person who doesn't even care for you. You can also do these things if your girlfriend is cheating on you.