How to calm down?

In the heat of the moment, just try to separate yourself from him and go someone else alone. Do lots of deep breathing. Long deep breathes in, slowly out (sounds stupid, but you would be amazed at just how much breathing helps in distressing situations).
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To try and prevent you building up all these anger and having outbursts, you need to find ways to blow off stem regularly. Like, sport. Running. Art. Music. Anything that interests you, that is generally physically draining, or self-expressive. It will take some trial and error but you know, try different things, see what you like, see what works. You could also try Googling suggestions as well. Don't bottle it up because that's the worse thing you can do, for yourself and those around you. You should also, when calm, talk this over with with your boyfriend. Have a word that either of you can say when you think the conversation is heading to a place that could anger you. This way you don't have to verbally say No or Stop.
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If this has been a problem for you for a while, you could consider going to some group therapy sessions. Group therapy has proven success for high-trait anger. Good luck and I hope you find a solution.

I have the same problem. I can never seem to hold my tongue, and I often end up saying hurtful things to the people I care about. So I started literally biting my tongue. Not hard - don't chomp down on it, just catch the tip of your tongue between your teeth.It took a little while for that habit to stick, but whenever I feel an automatic nasty thing coming out of my mouth, I bite my tongue. It'll literally stop you from hurting your boyfriend with your words, and you'll begin to recognise when you're about to say something and you can consciously stop yourself.Also, finding something that relaxes you can help loads. Try yoga, or reading a book, or I find the podcast 'Welcome to Night Vale' extremely soothing. Here's a link to all the Welcome to Night Vale podcasts, if you're interested: http://podbay.fm/show/536258179I hope this helps. xx.