if 25+ feels like the speed of light, I bet most of your issue is in your head. overly stressing, freaking out, or being anxious or nervous can easily increase your sensation of speed and make it very difficult to ride well. and of course, u cant expect your body to be relaxed and loose if your mind isnt as well.
as for grip, everyone has their own style so its hard to recommend something specific. just make sure you don't lock your elbows or wrists and you've adjusted your levers. the screwdriver method is mostly for hanging off really far and track use. so it prob doesnt apply to u now.
some awkwardness in trying to ride loose probably comes from how hard u are holding on. if u are gripping tightly to actually hold yourself on to the bike, u probably need to support your weight another way as Butch suggested. using your legs and core are the best ways to take weight off your hands. however, if you are gripping tightly because u are stressed, that goes back to my first point.