I want to practice guitar at night
Why don't you practice it at night then?
My girlfriend is always annoyed when I practice late at night
I see. That's actually pretty reasonable of her, since it's expected that most people should practice instruments during the daytime when others aren't trying to sleep. I think you should stick to practicing while the sun's up!
She's boring and I don't care.
I'm just saying that it'd be better for everyone involved if you practiced during the daytime. That way you won't be disturbing anyone.