1. It is most likely not about you.
I am in a stressful spot at work. You can't really have stress unless you have responsibility, and my current position involves the most responsibility I've had. And I am, relative to all my coworkers, a Dumb Guy. Thus, I constantly feel like I'm failing. But the thing to remember is, nobody is looking at me working and saying "man, this project is in a rough spot; must be because Dan is a dumb guy!" Everyone is on the same side. Everyone wants the project to succeed. Whether or not I'm a dumb guy is secondary, or tertiary, or whatever, geez we'll worry about that later, we have work to do!
2. Wanting things stresses you out.
I want some things, particularly at work. For example, a job that I love. But worrying about days when I don't love my job only stresses me out more. And whenever I set some target in my sights, I feel better after letting it go. There's something wonderful about wanting something but not really caring if you get it.
(I'm not sure how that squares with really wanting something. What if you were hungry and you really wanted food? You can't say "meh I don't really care if I get that food." But if you can choose to want something or not, it's nice not to be too fussed if you don't get it.)