God, grant me the serenity to accept the things I cannot change,

Courage to change the things I can,

And wisdom to know the difference.

If something sets you off you should try to identify why it sets you off, if you can. Think it over (if it is an internal issue) or talk about it if it's something that directly involves her or your living situation (you didn't explicity say you live together but it sounds like you do). Try to identify why it makes you angry and what you (or the two of you) can do to handle the situation differently in the future. Often times small things become big things because they get bottled up and fester and boil until they explode. Is it clothes being folded a certain way? Dirty dishes? Shoes being left in front of the door? Verbalizing and working through idiosyncrasies about little things can help a lot.

It is tossed around a lot (too much) as an excuse today but often times anger problems are rooted in childhood trauma that can be addressed by going to therapy. We're you abused physically? Emotionally? Sexually? Neglected? You don't have to divulge anything personal if you don't feel comfortable telling random strangers on the internet. That's what licensed psychotherapists are for: a literal "safe space".

Beyond that and the breathing exercises others have already mentioned: go run around the block (literally). Any time you get angry about something, do a mini-workout. Right then and there drop what you're doing and ru around the block or do 10 push-ups 10 sit-ups 10 jumping Jacks etc. A little endorphin drip can go a long way to clearing your head.