I tried all kinds of exercises, terry grips, a good one was a bit of broom handle with a string with a weight on the end, twist the handle using just your wrists to wind the weight up, then the other direction to lower it, hold your arms out in front of you.
The best thing was just riding more and releasing the vice like grip on the bars, witch also comes with confidence witch comes with practice.
Going to places like Glentress (not that ive been there) or trail centres, once in a blue moon and expecting to rip and shred all who lay in your wake just isnt going to happen, its easy to try and over do it, go to fast stress yourself out and end up riding slower, out of control and putting yourfelf at risk.