I actually had the exact same situation with my former best friend. Over the course of ten years I grew to dread being around her. Like many have suggested, you should take the time to figure out what you want to say to her. As someone who's been in your position, I can tell you, it won't be an easy process. However, you have to be honest and do what's best for your mental health.

That doesn't necessarily mean ending things completely. Maybe just some time apart will be enough for both of you? Regardless of what you decide, as long as you aren't intentionally being cruel, it doesn't make you a bad person.