It is the pucker factor induced by a hardshell. Up until now, you have been in a boat that stays right side up all by itself. Now you are in a boat that stays upside down all by itself. I bet the added stress of keeping yourself upright translates into several things, the 1st of which being tenser muscles and grip. Hopefully more time will help you relax a bit , more. Trying the straight shaft does sound like a good control. Also, are the blades of the new paddle similar in size/shape to the old one. That adjustment could make a difference too.
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