I know exactly how you feel. When you're not sure what you should be doing with your reins, the temptation to fiddle is just too much, and I was the world's worst, and the world's worst worrier as well!
The Libby's synthetic reins with rubber undersides pretty much cured me - at least, of worrying. They're lovely and light to hold - not a great fistful like ordinary rubber reins; also, I found the ones with stops made me fiddle even more because I was always counting to see if I had the same number of stops in one hand as the other!
The Libby's don't slip even if the horse/reins are wet through. That illiminates any *unnecessary* fiddling, which means that you can afford to allow your hands to be purposely a bit "mobile" as I think Reiner Klimke would have put it.