I'm going to break up with my girlfriend.
Really!? You are going to break up with her? Why?
She wouldn't stop talking to me about irrelevant things.
Have you tried telling her that she's way too talkative? I'm sure there's a reason behind it, and think of all the things she is making you happy. I am afraid that you may regret breaking up with her just because of something that you find irritable with her.
She messages me everyday and I'm sick of it.
It does sound like you are not liking this relationship anymore and it is understandable that you want to end it. Just try to be nice when you break up so that she doesn't end up in a lot of pain.