If you are romantically interested in her, then it isn't really a friendship. My friends call this Standard Vulture Position. You're just waiting around to see if her current relationship will drop dead, so you can swoop in for the remains. It's creepy and disrespectful to both you and her. If she really, truly wanted to be in a relationship with you, she would break up with the other guy. But that isn't happening, and her "mistake" excuse is b-s. She sees you as second choice (if that) compared to her boyfriend, and she is only keeping you around as a spare tire. Might as well climb in the truck of her car and lay there next to the tire iron. Don't be a vulture or a spare tire, be an assertive human being. Tell her what you want: a romantic relationship with her. If you can't have that, be honest with yourself and with her stop being her fake friend. Move on and look for somebody who is available and interested in you, and doesn't see you as second-best or worse.
Yes, you will have memories of her. And those memories will fade as time goes by, once you move on. I had some hopeless crushes when I was a teenager and even in my early 20s. To be honest, I remember all their names and faces pretty well. But the feelings aren't there, because I moved on and had real dates and real relationships with women who chose me. And those are stronger memories than the ones of the women I didn't get to date.
Good decisions come from experience. Experience comes from bad decisions.