Letting off some steam

There are very few people in my life that I turn to when I am in dire need of letting off some steam. There aren't very many instances where I feel like I really HAVE to do this, but when I do, I feel like I will go insane if I don't.

Everything will be going smoothly, until suddenly something happens and my mind goes haywire. It will be the tiniest of things that set it off too, which makes me even more livid afterwards! Suddenly, my mind is going through all the things I may have done to deserve it. I go through all the options and settle on one that I think is the most plausible. That thought then turns into my reality, and I start to believe it. And once I believe it, my blood boils because I start taking my anger out on the person/situation. I start creating conversations in my head and responses, which I believe to be real replies! And it's just a downward spiral from there. All this happens within 2 minutes after the trigger. 

I hate this because I make up a huge mountain out of a very tiny, minuscule molehill! It makes me feel and think things that I immediately regret! Especially when the situation takes a completely different turn than I imagined (which happens 99% of the bloody time)! 

How do I turn my mind off? How do I tell it to shut up? To leave me alone? But I'm pretty sure if it keeps making me feel like this, I'll have to be admitted to a mental asylum.

"And I will stumble and fall
I'm still learning to love
Just starting to crawl"