I think I want to break up with my girlfriend but I am not completely sure.
What is your reason for wanting to stay and not break up?
Because I'm afraid of being alone. Without her I'm sure I'll be miserable.
Since you acknowledge that you'll be lonely without her I think it means something else is going on. You should try couples counseling to see if it can help with whatever problem you two are having. Hopefully you can resolve it and move forward happier.
It was just a thought I had but I'm not doing it.
Well if you are having thoughts of breaking up, talking to your partner to sort troubles out in your relationship could be a sound idea. This will allow your relationship to grow.