How to leave my best friend without being rude or obvious?

We went through high school together and now we are doing the classes in college. Its not that i dont like her, ive just grown bored of her. We are eachothers only friends in college so we kind of only hang out so we arent on our own. we dont hang out on weekends or anything like that. But Id rather stay on my own, It doesnt feel good with her anymore. But she doesnt feel the same. I try to back off and hang out with a new friend i made that i like being with, but she gets annoyed when i do. And shes always with me. Even if me and my new friend are talking and she feels left out, she still sits and listens. and then afterwards makes fun of her and what we talked about. I dont want to just ditch her, i dont want to be the evil best friend, she was there for me when i just started school and had no friends. I want to be there for her too, but its not fun anymore. I tried gradually pulling away but its not working. The truth would hurt her. I dont wanna lose her, i just dont want to be always with her. How would you go about it?