When I was younger I identified as bisexual. I came out to people and it was easy to say "I'm bisexual" out loud. I'm not actually sure if I like guys. I don't actually want to be bisexual. I just want to be a lesbian. I find it hard to say "I'm gay" or "I'm a lesbian" out loud.
How often do you find yourself needing to say either of those things out loud? More importantly, you say you WANT to be a lesbian. Are you saying you DON'T like guys, or you just don't WANT to like guys?
As Ellia asked, how often do these thoughts occupy your mind? The more you think about it would be an indication of much your true self wants to manifest itself. Also, the closer you get to the truth, I find, the more these thoughts occupy your mind until they become truth. Sort of like a metal detector. The more you search and the closer you get, the louder and more frequent the beeping. You need to look at this objectively. Firstly, have you had both a relationship with a girl and with a boy? I’m an ardent believer in doing is knowing. You say you want to be lesbian, but aren’t sure. The only true way, in my opinion, in being able to know, is to try a relationship with a girl. Or at the very least, imagine what you might feel, really, if you were in such a situation. If it’s just a phase, then satisfying the curiosity should satiate the desire to be exclusively with women or be attracted to both men and women.