My girlfriends best friend is doing her best to break us up?

so heres the situation... for a while i was unsure of which of these two i wanted to date one had a great personality while the other was just stunningly gorgeous and they both liked me... over time the grogeus girl became more fun and open. so i dated her. at first the other girl was very supportive as in she helped me ask her to homecoming and everything. after we actually started dating however she started getting really nasty.. at homecoming she attempted to get me to cheat on my girlfriend with her. this girl has alot of power over my girlfriend and lateley keeps convincing her that im bad. i have to drive down to her appartment and convince her im a good guy again wich is really annoying. the most recent thing she did was convinced me that i was slept with this other girl that gave me a ride home. the worst part is my girlfriend actually sort of believed it. it doesnt seem like theres any option left because it wont be long before she makes my girl choose... i will lose because they cant live without eachother "so she says" i know this is an unhealthy relationship but convincing her of this is impossible. please help so far what i have to help is she (has a boyfriend now and her happiness totally depends on how they are doing) (she is overprotective of my girlfreind because of all the bad things that have happend to her before) and i also think she thinks that i am trying to replace her and im not... id give it two weeks before the ultamatum occurs and i really really like this girl ANY advice would be wonderfull and thanks in advance