I no longer like my best friend
What happened to end the friendship so quickly?
I have found somebody else I care about more.
Friends can sometimes grow apart so it is normal to not want to be friends anymore if you don't like them anymore. It is okay to tell the friend that you don't want to be friends anymore.
I just want her to keep quiet about me.
I think that ending the friendship is okay as long as you give explanation. It is okay to end friendships with people if you do not feel the same about them anymore.