It can definitely be easier to stay friends with a guy if you are also friends with his girlfriend. The thing is, if you start asking too many questions about their relationship, she might see you as a threat, or think you might become a nuisance to her. I know the situation sucks, whatever you do. Hang in there. Don't be afraid to put some distance between you and them, to let your feelings fade.
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"…it is the struggle itself that is most important. We must strive to be more than we are, Lal. It does not matter that we will never reach our ultimate goal. The effort yields its own rewards."
-Data, android, Star Trek TNG, describing becoming more human
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