I love my boyfriend but I'm a lesbian.

I need some help/advice. I love my boyfriend so so much and I see myself having a very real future with him. We've been together for 2 years and are serious. But I'm a lesbian. I know it. I've known it since I was in the fourth grade and I've been pushing it down and around ever since. I haven't really ever felt trapped or anything like that. Just enjoying people and trying to be happy. I've been with women almost exclusively before meeting the guy I'm with now. But I don't know what to do. It seems like the more time goes by and the more serious we get, the more I panic a little because I don't want to be 55 and finally come out and wish I had done it now. You know? I've asked him already about a threesome, just so I could see if that's all I wanted or if this is more serious, but he doesn't want to because he's worried I might leave him for a woman if we did that.