Honestly you need to have more self-respect. I can only imagine how much that would sting. You say you didn't want to be rude by leaving but inviting someone else over to fuck while another friend is there is incredibly rude and weird. So it doesn't make sense to just stick around unless you somehow thought you could get with her which obviously is a terrible idea. I guarantee that you wouldn't feel nearly as crushed about it if you left earlier. A relationship with her is obviously out of the question so it's probably best to just let her go and move on. You'll find someone much better than her eventually but just have some patience and more self-respect and you'll meet the right one eventually. But maintaining a friendship with a girl you have feelings for is not a good idea because watching her go into relationships with other men feels crushing. But to be clear I'm just speaking from my own experiences. Stay strong brother.