How do I stop my angry feelings?

For the longest time, pretty much my whole life, I have been excellent at keeping all my anger inside and suppressing it, dealing with problems and disagreements in a calm, rational manner, always one to say, whenever someone asks me "sure I feel angry sometimes, but there are ways to sort out issues other than arguing - calm discussion, etc." and this has been my way of life for as long as I can remember (I'm 28 next week). However, for the past few months, I find myself noticing more and more, problems and issues around me, and in my head, I mentally explode at the people concerned, and imagine myself arguing them into total submission. It's even got to the stage where I am now doing what "everyone" does and snapping at people when I feel annoyed. It is only recently I have noticed this change in me all of a sudden, after I did it at my girlfriend, who doesn't deserve that at all. I want to go back to how I used to be, it's how I was happiest, how I'm comfortable. So does anyone have any advice on how to deal with this sudden increase in destructive anger? I hate arguing and snapping at people, and I wasn't always so weak that it kept getting to me, and I don't know what is different. My girlfriend says that I am the "water to her fire", and I want to stay that way - too much fire is dangerous. Can anyone help me?