I have a crush on one of my girlfriend's best friends
That's complicated. Why do you like her and not your girlfriend?
Because I think my girlfriend is plain boring. Her friend is so much cooler than her.
Crushing on someone else while in a relationship happens all the time. But it will ruin your current relationship if it goes any further.
It will be fine. I don't think my girlfriend likes me that much anyway.
If you two aren't into each other then you should break up. That way you can be with her friend instead.