Periodically, I find myself feeling nervous for no apparent reason, or sometimes for a reason, at which point my mind will create stories to instill fear and worry into my being.
My stomach would tighten up and when observed closely, I could feel myself breathing shallowly.
It is so easy to fall into that state once it starts.
Your mind is so powerful.
It is so easy to be unconscious.
But you know deep down that you are in control and that you can switch out of that state in an instant: the instant you decide to feel joy and refuse to believe in your mind created story, which is no longer serving you.
Three powerful questions to ask your self at any point:

1. Am I breathing?
2. Am I relaxed?
3. Am I moving with grace? (If you are walking)

I picked up this beautifully simple yet effective technique from Dan Millman.
The beauty of this technique is that, we can consciously and fully focus on one thing at a time, and so when you have 3 things to focus on, you will be thinking less of your problems, your worries, your stories.
I've used this simple technique in my photography work to help my subjects relax, and I've suggested it to friends they are buried in worries.
It's very effective.
Suggested book: "Way of the Peaceful Warrior", "Ordinary Moments: A peaceful warrior's guide to daily life".
I've read both books, and they have brought peace into my life.
Peace and better understanding of myself.
If you only pick up one copy, get the first one, it was one of the core books that changed me for the better.
I hope they will contribute into the wellbeing of your soul as did mine.