I am sorry that you are hurting.
But if you really want to leave that door open, bear in mind that you will invite all of this quite difficult past into your new relationship with him. I don't think that you will manage to stay friends with her. She doesn't sound like she would be able to tolerate it. He and you will always have to deal with the way things went with her and you two.
As you said: how to un-love someone? You will hurt from this if you stay involved, even if it's just with him, because she can't be part of it as well but will always be on your and his mind. For your own emotional health, make a clear cut, hurt as much as this situation will inflict on you and try to find another love that is free of all this drama and burdened history.
I hope you can figure it out. I know logical advice tends to be easily given and how difficult it is to rule your heart out of the equation. Despite that, good luck.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.