Wrists hurt = overuse. Take a break.
Example, I used to do 10-15 rounds every day on a 200 lb bag, loved it. Knuckles grew big and ugly, callous you could put a cigarette out on, would tear it off the chain holding the bag from the cieling support once a week after a while. Then I noticed a couple of nasty things:
I developed a very tender spot at the heel of the palm in the wrist where the gap is between the bones of the hand and the bottom of the wrist.
I began to devlop what I believe now were the equivalent of shin splints or stress fractures in my forearms.
Now I love working a bag and the ability to work one for an hour is a fine thing. On the other hand, I lost mobility that took me a long time to get back and dexterity which is still shot doing this. i still pack a mean punch, but know better than to play boxer all week long now.
If it hurts that much you are complaining about it rather than grinning about it, knock it off a while. A wrist will benefit from the time off. So will your psyche when you come back to it and the skills are still there.
Keeping the knife stiff will not increase the use of it unless you are thrusting into hard targets, I have never even considered it. Take some advil, have a cold one and go for a run instead. Take a month or so away from the blade and do other things not involving lots of knifey movement, and then get back into it slowly, like an old pro coming back to it for the enjoyment of the blade instead of the necessity.
