Could I possibly be lesbian?

"I mean I know I'm bisexual."
Hmm....you're probably not lesbian.
Bisexual isn't 50/50 like how people think bisexual would be. Sometimes you're more interested in one gender more than the other (in this case, of the female sex).
I'm the same, my mind defaults towards men and while I didn't date men as much, my relationships with men was a lot better and exciting. When I do meet a woman that I love, I do think about her a lot more than men. If you think about men at least sometimes in that way, you're likely bisexual.
If not....you might just be a bi-curious lesbian. I wouldn't really know. They're all just labels anyhow. Don't worry about the labels, just let things go naturally. If its not possible to be happy with your man, perhaps he's not the right person for you or you're not the right person for him. It's best to discuss it with him, he might just help a lot better than me.