Dude, you need to grow up. If you want to be with this girl fine, be with her. There is a cost and it's knowing that you're going to hurt and lose your best friend. It's as simple as that. That's the price and if your willing to pay it, tell her to break up with your friend and start dating her.

But you need to quit this whiny "I can't help how i feel, it's not fair, this is true love" shtick. 

We all have things that we want that we can't have for one reason or another. You can choose not to act on these feelings and there are things you can do to distance yourself from them and move on from them. But what you're doing right now, coming on here and trying to find some validation for doing a shitty thing, well, let's just say that it show's a lack of maturity. And you you know what? You're 18, by definition your immature, so instead of wrapping yourself up in this melodrama, why don't you step back and use this as an opportunity to learn and grow?

Use this as a way to be a better person and a better friend. Learn that you can live with some hurt and some wants that you can't have. Learn about setting boundaries, not because you want to, but because it's the decent thing to do.

If nothing else take time to reflect and decide what sort of person you want to be. Because this is a moment that you'll look back at in 10/20/30 years and can either be ashamed of or proud of.