SO LOST, I think its right, but its so wrong, love, friends.. betrayal?

there is my best friend and i love him. like not just best friend love but I love him. He claims he loves me. we do everything a couple does and he tells me how comfortable he is around me and how his heart beat picks up every time. Its just he has a girlfriend, my best friend. They've gone out for a month and are best buds, which he realizes but still really likes her. I can't tell if he likes me more, but he says he LOVES me. Idk what to do we've been sneaky behind my friends back and I can't keep doing this. HELP!!