My girlfriend has had a crush on a friend of hers for years and has never gone out with him only because he has never expressed any interest in her. She only told me this about six months after we began a relationship, and it hurt me because it made me feel as if I was a back-up plan. She has also told me that her feelings for me are genuine and that she wants to be with me no matter what, but a part of me keeps thinking about what would happen if this guy were to change his mind and ask her to be with him. I know this is unreasonable, but I feel insecure and am constantly questioning everything she does since we had that conversation. How do I get past this? I love her a lot and don’t want this to end.
Why not give her the benefit of doubt by accepting what she has told you, for a start? She says she wants to be with you and has chosen to be with you for all this time presumably because she wants to. She hasn’t been forced into this. She may have a crush on someone but has also told you about this because she feels comfortable and secure enough to be honest with you. Your relationship can end for all kinds of reasons, least of all the presence or absence of someone she has a crush on. It makes sense to focus on what you have and how you can make the bond between you stronger, rather than worry about what may or may not happen. Being together is an opportunity to try and make this work, which is all you should focus on.

Is it okay for me to end a relationship just because my partner and I don’t share similar interests, or am I being shallow?
There are all kinds of reasons why relationships work or fail, and this depends entirely upon the importance of those interests to your life.