First, try and remove those things that are stressing you. If it is your work, find time to take a break and go outside, sit or stand in the sun, take a 10 minute walk around outside or whatever. Sometimes just a change in the scenery helps.
If I can't remove the stressors, I will calm myself down with music and breathing techniques. Choose music that is calming or played by an instrument you enjoy playing or hearing - a piano, a guitar, a flute, a harp - one that makes you smile, dance a little, think of a different place, a happier place, where you first fell in love with this type of music or instrument.
As for breathing techniques, take a deep breath in through your nostrils, hold if for 3 seconds, then exhale out your mouth. In your mind picture that you are bringing in good clean air through your nose, good clean oxygen to clear your brain and give you energy. Hold it i for 3 seconds then let it out through your mouth expelling with it all the negative thoughts and negative energy with the air.
Next breathe in again and tell yourself that you are good, you have the skills and talents to do whatever you need to do in this place and time, hold it for 3 seconds, then exhale out through your mouth all the thoughts that say you are no good, you are not important, you cannot do this job.
Try this a few times until you feel like you are either ready to burst or fall asleep. If you are ready to burst, you need to do some more breathing exercises. If you are ready to sleep, you are calm and can now focus on the task at hand.
If you are a Christian, you can also be praying while you are breathing asking our Father God to support you, be with you, give you his Spirit to calm you.
I wish you well. And would love feedback on whether any of this worked for you or not.