So sorry you have to go through this. It really must feel like a second bomb being dropped on you. And the so called BFF cannot even be considered a friend. If she truly was your friend, then she would have never imagined anything with your H, even if she had any kind of feelings for her. I'm so sorry you have to lose "a friend".
Don't feel guilty for all your anger. I would be furious if I was in your shoes. Unfortunately, anger does not resolve any problems. You will have to go through the grieving stages of yet another loss and feeling angry is part of it, just try not to let it out on your H or OW.
Visiting your grandma would be a great idea. Take time to get away, calm down and think things through.
