You know, these things tend to take more time than a week. There are people at 40 who are still figuring out their orientation, so your age is somewhat irrelevant here, and in the great spectrum of things, 18 is pretty young, especially if you still havene't even had more than a couple different relationships with people of any given gender, especially the gender you feel you're most attracted to.
I'm not telling you you aren't a lesbian, because I can't know that. But what I can tell you is that not feeling able to date one specific boy isn't what I'd call a sound basis for deciding you *must* be lesbian (and remember, very few people fall squarely into heterosexual or homosexual mmodels, anyway -- most people in the world are somewhere in between).
What's the reason you can't be with him? I can assure you it isn't about being queer primarily, because even the strictest of lesbians can find themselves falling in love with someone who isn't female. It's simply because as it turns out, you don't have strong enough feelings for him, that's all, and potentially because right now, you're feeling out your own sexual identity and dating boys is confusing you further.
What I'd suggest you do right now is slow the pace, a lot. Before you go freaking out, how about taking the time to read up on being queer?
A couple good books to start with might be: "Free your mind:the book for gay, lesbian, and bisexual youth--and their allies" by Ellen Bass and/or "Hearing Us Out: Voices from the Gay & Lesbian Community" by Roger Sutton.
Editor and Founder, Scarleteen
My epitaph should read: "She worked herself into this ground."
-- Kay Bailey Hutchinson