I'm thinking of breaking up with my gf
Why would you want to break up with her?
She's the most annoying person I've known in my life.
If you feel that this is the end of the relationship, you should leave. It is good that you are not leading her on at least.
I'm planning to do it soon, but I don't want to hurt her.
It's good that you want to let her know soon, because stringing her along would be rude. I'm sure you can figure out a way to do it without hurting her feelings.