I think that you’re trying to preserve a situation that can’t be preserved. If you want a lesbian romance with your best friend, you really need to find out if she’s open to that with you. If you don’t find out, you’re going to make yourself crazy. I can’t tell you whether or not she’s a lesbian or open to that kind of relationship with you, so you have to do the hard work, risk rejection, and let her know what you want. She can’t read your mind.
In addition to which, you really need to break up with your boyfriend if you have such strong feelings about someone else — man or woman. It’s not fair to him, and it’s not fair to you.
Don’t let your fear of being alone keep you trapped in a relationship you don’t want to be in. Being alone is an opportunity to recreate your life and see what else is out there.
So bite the bullet and talk to your best friend about your romantic feelings towards her.