My lesbian friend thinks I'm straight, but I have a crush on her, and it might ruin our friendship?

I really love her, genuinely, and I know she loves me too (at least as a friend). I know she would date me if I asked her out, but once I take that step, there's no going back. I will have to either be her girlfriend until one of us moves away, spend the rest of my life with her, or break up with her, which would destroy our friendship. I'm afraid to ask her out, because it would change everything, forever. I have had two girlfriends before I met her, and with both of them, I found I was just bored of them after about a year, and I broke up with both of them. I don't want this to happen again, because I don't want to lose her, and I don't want her to lose someone else she cares about. She doesn't have a lot of friends, and her parents were killed in a car crash six years ago. I can't let her lose me, too.