The muscles in your hand and fingers are small and tire pretty easily. The muscles in your arm and shoulder are bigger and can do a lot more work. Back in the day, folks who were taught "business cursive" like the "Palmer Method" were taught to keep their hand mostly static, and to guide the pen strokes using their arm. The way I do it is by planting my forearm on the desk as a pivot point, and moving my arm and shoulder to make the pen move. It took some getting used to, but my handwriting looks about the same as it did before, but no more hand pain.