that i know my girlfriend has a boyfriend, and i'm just going along with it because loneliness is even worse.
Might be a bit late, but I was in your situation before, and I can safely say you will feel really free once you leave someone who doesn't respect you. I feared it too and pushed a relationship with literally and seriously the dumbest person I've ever met for over a year, damaging my idea of love and the role of relationships.
It is so, so much better to just dump her. Get it over with and you'll feel free.