Oh honey I'm so sorry. I'm so sorry.
I wish I could say something that would make this easier, but the sudden violence of this comes through clearly in your post; I will give you the best advice I can summon, piteous as it might be against what you're going through. I am so sorry and if I could offer you anything more than this to comfort I would do it in a heartbeat. Hold on to your heart, and understand you've been done a bad turn today. I am a sometimes lunatic woman myself, and it's from that perspective I condemn her.
She treats you terribly at this moment. You don't get to do what she's done: once you make a vow to a man, that fucking means something, something that should chain you vein to heart to this man for as long as you can live. That is the promise you make. You don't get to make that vow lightly or climb the church steps unless you fucking mean it to the core of your soul to this one man alone. She does you a bad turn today, she acts like a child at your expense.
But I know what it is to love someone who does something like this.
In this moment of her madness, if you choose to act with love of her, let her go. If that's what she's determined herself to do, even if you stop her or hold her back, she has hurt you unforgivably and the trust has been cracked. It is best to let her do what she's going to do and take this time to nurse this injury.
You will survive, I promise you, I can read a good man even in these few lines and my heart absolutely goes out to you, life is not over tonight. Breathe.
Steel yourself to let her do what she'll do. You will need to conserve your strength for the days ahead. Focus on them, not on this, not yet, not right now. Just breathe, right now, and let this girl do what she wants to do. It's probably even better that she goes in the grip of this sudden viciousness. If she really was in love with the fucking guy she had no business making the promise of matrimony to you. You are entitled to anger, you have lost no honor in this thing.
Once she goes, if you follow my advice and let her, you will have a few days of pain, a few days when you feel like your heart's coming out of your body. Breathe. You did no wrong and you loved more, that says the world for you. Collect yourself, examine the situation. The rest of the process I leave to you; listen to your wisdom, not your pain or your desperation. You are a good man. The world needs more of those. Don't let this make you turn against yourself; you did nothing wrong, you didn't cause this. She made you a promise in bad faith. You have good cause to feel however you feel. Whatever you experience, don't let guilt in.
And honey, you'll live through it. Let her go, and make your decision in some quieter moment what you want to do with the girl from there. You've been done a bad turn tonight and again my heart absolutely goes out to you. I wish there were more I could say, but the most imperative thing is that you did nothing wrong. Love is your power; don't let this take it from you.