A Really Long Time

So I've been in love with the same guy for thirteen years. It was just a crush when we were kids, but now we're adults, I'm in love with him, and I've never told him. He has a girlfriend now, though, so I can't get it off my chest. I know he'd never feel the same way, but that's not enough closure. I can't have any other relationships, and my feelings are killing me. What do I do?