I'm going to break up with my girlfriend and start to date her best friend
Why do you want to break up with your girlfriend and start dating her best friend?
She is prettier, younger and a better catch
I can appreciate you wanting to date a prettier girl, but I don't think you've thought this through completely. There is a good chance that both of them will hate you if you try to dump one for the other. If you really feel this way, I'd recommend ending your current relationship and waiting a while before pursuing something with the best friend.
I feel like I'm a man stuck between two women.
You may end up like a man stuck with no women. Her best friend may not see you in very positive light if you pursue her. Have you presented her with your scheme?