Please help my relationship!?

Where do I start? Well, I've been going out with my girlfriend (Alex) for 7 months and I have recently become really good friends with her enemy(Eleanor). Now she suspects me of cheating on her with Eleanor and her trust for me has weakened, we are on the verge of breaking up and she knows that that would break me heart. I really love her, more than anything in the world and I know she loves me just as much. She cries herself to sleep every night so i really want to fix it fast...What should I do?