How do I fight off jealousy?

A guy that a really really liked just started “dating” my best friend. They haven’t actually gone out yet because she just met him a week ago but I saw her sitting in his lap and they were holding hands when they thought I couldn’t see. She obviously likes him back, and I’m happy for her, but I can’t help but to be jealous. She doesn’t know how I feel. What can I do to prevent jealousy, or just prevent showing it?