Or God exists and everything does matter.
For me, it helps to remind myself that I can only achieve so much in a given amount of time, and to just pace myself: better to slow down than to move like the Flash and produce sloppy work.
Also, too often ppl forget to breath properly. It may seem insignificant, but shallow breathing tenses up your whole body making you feel more stressed than you actually are.
Alot of times we're not even stressed by the circumstances around us, but by our own expectations. So don't put so much pressure on yourself, you prob have higher expectations on yourself than your employer does.
There really isn't much more you can do. If your talking about non-stop stress, you should prob consider a diff area of volunteer work cause some people are just completely unsuited to work with handicaps. No offense to them, but I have a cousin (love her to pieces) who is severely challenged and it is VERY exhausting being around her for prolonged periods of time.