Um, what? No. NEVER a loose grip. I think a better description would be "firm, but relaxed." Have a solid grip, but don't white knuckle the bars and keep your arms and body loose, but not your fingers. I've seen a guy with a "good but loose" grip take down more than half of a racing peleton on a 30+ mph downhill when he hit an unseen bump and lost his grip on the bars.

OP, also try moving your weight back a bit on the saddle when descending, practice cornering on the flat, try and find some short hills with a turn at the bottom so that you can practice and build comfort in a predictable way.