If it's hard for you to tell her directly, then you can not do it. Then you can gradually reduce your communication to completion. Pretend you're constantly busy and don't have time. Stop sharing emotions, thoughts, secrets with her. I think that in time she will understand that your friendship is coming to an end.

Of course, this will not be very good for her. It would be much fairer to just tell the truth. But you have to choose how to do it. And you decide whether you want to hurt her feelings or not.

But if your friend is really toxic, it is really worth ending the friendship.