I think I'm going to stop talking to my former best friend.
How come you never want to talk to them?
I found out that she's been saying mean things about me behind my back for years.
If it is true that she's been talking behind your back then you should stop being friends with that person. No one should have to put up with friends who are toxic.
I'm going to get back at her by making her look like a dirty slut.
I know you're feeling upset, but getting revenge on her is a bad idea. If you try to get back at her, you'll just be stooping down to her level and it will take you even longer to make your peace with the situation. Just put her in your past and move on with your life.