Okay cheating can be so many different things. Are you talking about whether she emotionally cheated on you or like physically got with your best friend because um you didn't really explain that but regardless I feel like you should not give her another chance and honestly with your best friend I I feel like that relationship's never going to be the same anyways between you and him I'm really sorry this happened and I hope you're okay.
There's nothing wrong with forgiving. It's so that you yourself can move on and heal from the pain of what you went through. However, just because you forgive them doesn't mean that you need to stay in that relationship or have anything to do with them whatsoever. You can forgive and let go for your well-being, but that does not mean that they have to be in your life anymore.
Why would you ever ever get back together with your ex? The whole point of having an ex girlfriend is to learn from those experiences and not repeat them. If you just go back with your ex you are also the problem and getting back with her just because she's bad will just bring you with a lifetime of pain. Move on.