I'm in love with my best friend who happens to be a good friends girlfriend
That's such a problem, does this make you unhappy?
I'm unhappy with my best friend because he keeps giving me mixed signals.
I know that you can't help who you're attracted to, but this is your best friend! You can't betray them like this just for a girl, and home wreck their relationship. You need to keep your feelings in check and find a different girl.
But I don't want to let her go. I think I'm in love with her.
I would take a step back and reflect upon your friendship. Love is often fleeting, but a friendship will last a lifetime. Let her go.