How do u think the "3rd PARTY" in relationship???

The girl is staying with him? Do you mean they're together? Or do you mean living in his house? Either way, I think you should leave. I know how hard it is to leave but not leaving shows your boyfriend that what's happening is okay with you. If you leave and he chases after you and asks for you back, tell him to be faithful first. Tell him you won't take him back unless he can break up with the other girl he's seeing. This situation is unfair to you. It may seem like you're boyfriend's in control but the truth is, you are. You can leave anytime you want and he can't blame you for leaving if you ever do. Staying will only hurt you more. If he tells you to give him time, tell him you can give him time without being his girlfriend. You don't have to be together for him to be able to figure things out. This way, he can have his time and you can put yourself together for when he decides to stay with you. If he doesn't, then you'll have started the healing process already while he was out there having his "time".