This is going to sound really, really stupid but closing your mouth and breathing through your nose seems to allow me to calm down and focus better. Then when you’re riding, put a lot of faith in the bike. Bikes can roll over and cope with stuff more demanding than maybe you think they can. Let the bike do a load of the work, stay loose and if needed, edge off the back of the bike a bit, maybe even dropping your seat too. Death grip and hanging over the front of the bike doesn’t work.
The setup of your bike might need to change as well. Shorter stem/bars with more rise, forks too soft and diving through the travel, steepening your head angle?
I’d be inclined to agree with some others comments about a skills course. I went on one a few years back and from a purely personal point of view, I felt I picked a few things up from it but didn’t especially get vfm. What I did like about it (and maybe this would be better as a start) was that it was getting out into the woods and sessioning stuff over and over again, something I rarely do when I’m out riding. So maybe a more cost effective way of getting ‘skills training’ would be to go out with a better rider and find something (like this descent you mention) and session it, thinking about why it feels wrong and adjusting your body positioning and bike setup to counteract.