I'm going to break up with my girlfriend for her best friend
Why do you want to break up with her? Have you thought about how complicated this can get?
Because I realized I like her friend better since my girlfriend is always with her.
I understand how you feel, but don't break up with your girlfriend to go out with her best friend. If you do, it is likely that your girlfriend and her best friend will be unhappy with you.
It's time for me to inform them.
This is a very bad plan. If you don't want to be with your girlfriend you can break up but dating her friend is bad no matter what.