I think about cheating on my girlfriend a lot
Why do you think about cheating on her?
Because she's so controlling and I'm sick of it.
Having thoughts of cheating on your partner is not normal, and could make you start to resent your partner.  If your partner is too controlling for you, you should tell her and if she loves you she will alter her behavior.  If she doesn't change, maybe your not meant to be together.
I'm going to cheat on her with her best friend.
I don't think you should go and cheat on her, you should just leave her instead. At least you wouldn't be hurting your girlfriend as badly that way.