You might also want to try different combinations of gloves. Sometimes, the gloves have so much padding that they are forcing the diameter of the grip to be wider. This will cause fatigue more quickly in your hand/wrist.
Windsurfing had the same sort of problems and the industry standard now is to have skinnier booms made from tougher material (carbon fibre). The smaller the diameter of the object you are holding, the easier it is on your muscles/joint. Try gripping a pen. Then have someone try and pull it forwards from both ends of the pen. You will find that your grip is nearly unbeatable. Now try and grip a water bottle. Have someone try to pull it forwards out of your hand. It is much harder to sustain your grip pressure and requires much more work, thus you would get sore/tired much more quickly.
Keeping that in mind, if I were you, I would find the lowest profile grips (i.e. they aren't fat and padded), a set of gloves with less overall padding in the palms, adjust your levers so they are the most comfortable and use the palm exerciser.
You may also want to adjust your fork to deaden some of the high speed chatter that it seems that you are getting.
IMPORTANT: If you are already feeling a lot of pain in your wrist and hand, you may have damaged the tendons/ligaments. You actually have an injury, just not an obvious one like a sprained ankle or something. This means that you should rest for a while, maybe even a week or two to mend. If you just ignore the problem and try to 'tough it out', you may actually ruin your whole season and the pain will not go away. You could even do some serious damage. Joint pain almost always = injury. Use the RICE, (Rest Ice Compression and Elevation), method for a week or so and then build slowly. Do plenty of wrist/hand stretches and exercises.
Just my 2 cents.
Good luck.:)
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