My partner slept with someone on Friday. How do I cope? What do I do?

I'm so sorry for the pain you are going through. His actions were selfish and showed lack of respect for you. What you do next depends on how much you love him. Both of you now have to make an effort to make the relationship work. Understandably you have lost trust and are hurt. You can chose to try to forgive (which is possible after time eventually softens the pain) - forgetting is not always possible - but your partner is a human being and has made a big mistake and your love for him is being tested at the moment. Question is, is a one night stand worth chucking away your relationship? Is he truly sorry and explains to you why this happened and is willing to make amends, you can work towards forgiveness (after you give him a hard time of course - dont make it easy for him!). Good luck.