How do I tell my boyfriend that I'm a lesbian?

I’ve only ever dated guys.. I’ve never really felt anything with any of them, I didn’t know why. I’ve known for awhile that I’m attracted to girls, but didn’t think anything of it and just tried to repress it. Well, now I’m too a point where I can’t repress it. I’m coming to terms that I’m a lesbian.. Problem is, I’ve been involved with this guy since like last April. I care about him greatly, but I’m not romantically involved. I want to end things with him without coming out.. but I have no other reason to break up with him, so I kind of have no choice. How do I go about this, I’m lost on the matter. Any advice would be great..