I'm dating a girl but I still love my ex.

Alright, so I'm seventeen, about to be eighteen in December. I dated this guy my sophomore year for 7 months. ( I am now a senior) We loved each other very much and the relationship itself was the best I've ever been in. We broke up in the summer between my sophomore and junior year, but not because of issues we had. His mother split us up. It sounds insane, but it's true. She hated me so much and she didn't want me dating her son. I was torn apart, and I cried for weeks. I couldn't believe we were being split up for such a stupid reason. During my junior year, when we saw each other at school again, we talked again and gradually we slipped back into a romantic time. We never got over each other, in my eyes, and we dated in secret for about 3 or 4 months. We would act like we were only friends at school, but after school, we would sneak over to each other's houses and kiss and cuddle and have sexual contact. It confused me greatly. We never really knew if we were just friends or more. Now, currently, I'm dating a girl ( I'm bisexual), and I have been for about 4 months. We're doing really well aned I couldn't be happier, but I still love my ex. And he still loves me. I find myself pulling between them and I'm stuck. I really don't know what to do. I can't cheat. Cheating is one of the biggest wrongdoings I can think of. But at the same time, i feel myself being tempted to go back to him. Does anyone know what I should do? Any advice is appreciated.