I'm not physically attracted to my girlfriend anymore
I am sorry to hear that. Why are you not attracted to your own gf?
I find her plain and dull, she doesn't know anything I really care about
If you think you're not attracted to your gf anymore you shouldn't continue your relationship. It's ok to break up with her if you're not happy.
I'm ready to break up with her.
I think breaking up with her may be the best thing. You should let her move on and have a chance to find happiness and someone that truly wants her.