Don't Talk To Her Until You Come Out
First of all, does your best friend know you're a lesbian? If she doesn't, then you are right, you shouldn't talk to her about this until you know what to do with your feelings.
Define "In Love"
Feelings are interesting. When you say you are "in love," and you know that you and she can't be together what are you really saying? Are you saying that you find her sexually attractive and you want to sleep with her? Are you saying that the fact that she is not a lesbian makes her more attractive to you? If that's true, why?
We often say, well, "I can't help my feelings, I just fell in love." But, is that true, really? I think there are very good reasons why we fall in love with the people we do. If your friend is your best friend, then its natural that you would love her. Its a great thing to really connect with someone and know that the feelings you share are real love.
Your Desire Is Probably Not Appropriate
But romantic love is another thing altogether. Your desire for your friend is probably not appropriate. By sharing this physical, romantic desire, you risk losing her as an intimate friend, a confidant, a true love. Is that what you want?
If You're Just Horny, Don't Turn To Your Friends
If you think you have to mess up your friendship, question it! Why do you? What more do you need from your friend? Or, are you feeling a real deep longing for intimacy -- physical intimacy -- that is probably best satisfied with someone else? To put it simply if you're just horny, don't turn to your friends -- I think that's true whether you're gay, straight, whatever.
You are taking advantage of a whole set of feelings that is related to the intimacy and trust of platonic friendship and turning it into something else.
On the other hand, if the desire is really mutual, then taking the platonic love and merging it with the romantic love is this writers idea of heaven! So, I guess you have to sort it out for yourself.
Don't Tell Her If She's Not Likely To Return Your Interest
If you know she's not likely to return your interest, then don't tell her. Work it out on your own. You must have other friends, a therapist, a sibling, someone who knows you're a lesbian and knows your friend.
Oh, and I suggest looking for others to date and let the romantic feelings develop so that you can go back to being friends with your friend.