Classy, so sorry this had to happen to you. 'Part of life, though. I like the suggestions of tcklft, especially regarding the need to recognize and purge your feelings. I'd found through several cycles of drama with someone this year that my own heart was more and more inclined toward bitterness and suspicion with each successive turn downward, because of something they'd do too frequently that just doesn't fit in my definition of friendship. I forgave them each time, even the last, but forgiveness doesn't mean a friendship will continue, and it didn't.
It's natural and part of the human design to become bitter, I think, when your heart views a person or relationship as too emotionally damaging, as the emotions can become consuming and hinder one's performance at a variety of levels, thereby decreasing survival fitness. Bitterness inclines one to ignore or even treat the other with open disdain, offering a prickly emotional barrier to make sure they can't get close again or have the chance to impact your feelings, thus allowing you to go on with apparently minimal difficulty.
But I, too, have no desire to become bitter or closed off. Besides which, doing so does nothing except destroy communication and keep your emotions temporarily safe. Because someone else, sooner or later will find their way in there, and may again hurt you, I believe it's better to find a more creative solution for inevitable future difficulties than shutting down.
A certain degree of resignation is required, I think, and also evident in the advice some have given:
"6) Know that all things must pass, including what you're going through now."
"Perhaps simply discontinue your association..."
"4) After 3 days,consign both letters to your woodstove or fireplace..."
Discontinuing association is probably advisable for at least a time. Emotional distance is required for both parties to begin thinking at least semi-rationally and with more balance. Perhaps this time of separation will last forever, and perhaps not. Don't try to say forever or put any deadline on it, either. Let time and your hearts decide, but don't rush back too soon.
My own form of resignation involves this line of thinking:
All human beings are limited and flawed. It's just the way things are. They're going to screw up, and so will you. Accept and forgive both them and yourself for your flaws and limitations, because they are things you can't always see or always help. When you truly realize just how limited we all are, you can't help but be forgiving. And when you can forgive and forgive completely, you find it harder to be bitter, because it's no longer their action against you that you're acknowledging, but you're accepting their very understandable, very human limitations and their failure to see a better way. They are flawed, as are we all. Forgive them as you want to be forgiven by others when you've erred.
Using this line of thought consistently and often, I've managed to push my feelings away from bitterness and into a sort of emotional neutrality. Now and then, they still need a nudge to keep them there, but I keep at it, because I think it a worthwhile and good thing to do. And mind you, forgiveness doesn't mean forgetting. There are some things a person does after which, it better serves you to be wary. So let your rational caution and memory keep you safe, and your forgiveness keep your heart open, but calm.
I hope you can find peace one way or the other.
If you'd like to PM me on this matter or anything else, you're most welcome, Classy.