I'm really happy that my best friend is about to break up with her boyfriend
Is he that bad or do you have another reason?
I just don't want to share her with other people.
Yikes, I think your friend would probably be offended that you've found some silver lining in this situation. Additionally, you sound really co-dependent on your friend, and that's unhealthy.
I can't stand people being happy.
Have you ever heard the saying misery loves company? It is really unhealthy to try and make others unhappy if you feel that way. You need to work on yourself.