As a writer, you are aware of the power of words. if a thing is expressed in words it can become much more real, it is set down, immutable.
So perhaps think about how you wrote this question, what you set down:
Being a ... stressed out, and prone to anger person
Expressing this as your reality of what you are, rather than as a temporary emotion you are experiencing might be making it much harder to break out of the anger and upset.
If you always characterise yourself as stressed out and prone to anger, then you limit your own potential to be another way. If stress and anger are metaphorically on your business card and your office door, how you identify yourself to the world then you are more likely to default to those reactions.
So my suggestion is to come at you question from another angle, rather than ask other 'how can I write when I'm angry?', ask yourself 'How can I be less angry so that I can write more?'.
Anger is robbing you of writing time, perhaps remind yourself of that when you find yourself getting angry and stressed. When something triggers those feelings, ask yourself if the later loss of writing time is a price worth paying for that reaction. Use the desire for writing time as a tool to help you control the anger, to interrupt the processed that lead to you being stressed.
Other answers have given tips on relaxation techniques etc which I think are really valuable, but where possible I would suggest you shift the application of technique back to the point when you begin to be stressed and angry, rather than try and tackle things when the anger and stress have built up a good head of steam.
I can't recommend any specific anger management techniques, but if anger and stress are so significant in your life that you feel they define you and they interfere with your ability to engage in other activities, it might be worth looking for external support or counselling, if you have not already taken that route.