I know you're going through a hard time right now. You're questioning your sexuality and your relationships with your good friend and boyfriend.
There's a good chance that you're either bisexual or bi-curious. It's perfectfully NORMAL to be intimate with a member of both sexes. That doesn't mean you're a lesbian though.
Based on your feelings towards both of them, it appears that you're bisexual.
I'm NOT going to condone or condemn your drinking. However, I'm confused if the alcohol is meant to cover up or the cause your intimacy with her. Or does not the drinking have no significance?
Have you talked to her about your feelings?
Please consider talking to her if you haven't already. Best of luck