So to calm myself, I first try to change my environment. Go to another room or outside. I concentrate hard on looking at things around me and really thinking about that thing. For example I will look at my shoe, and notice the brand and color. I notice that it's looking pretty worn, but it's my most comfortable shoe. (For this foot.) I know I should probably let it go,but I haven't been able to find one as comfortable.
You see? I am totally not thinking about what I was so emotional about and when I get back to that, I will be able to look at it a lot calmer and more rational than I was.
You don't have to stop there, but everything you concentrate on, give it your full attention. Look for the smallest, least significant thing and describe it to yourself. Make it significant.