I think I can help you.

DON'T PANIC

If you mess up, move on, and fix it next time. If you start to beat yourself up over every missed shot, and misfire, you'll expend all of your energy worrying about what went wrong. While you're doing that, you're bound to bugger something else up, then you're really in the weeds.

How not to panic:
Practice. You have to go through the motions, a lot. Start slow, beat on the dummy for a good while, maybe even until you figure you can DPS it in your sleep. Do a bunch of dungeons, see if it made any difference. Once you're comfortable with the motions, you can work on the "harder" stuff.

From your post I read that you're working on everything, all the time. Life in the fast lane isn't for everyone. It's easier for most folks, men especially, to work on one thing at a time. Sort out the basics, then pick one thing you know you do poorly, and work on that. Once you're not panicking over that thing, choose another. Before you know it, you'll be a calm, cool, and collected DPS powerhouse.