Something against hand pain due to writing?
If your hands start hurting or feeling sore, take a break. Listen to your body - if something hurts, it needs to rest. Otherwise, you can risk permanently injuring/disabling yourself. One artist whom I follow, Yuumei, has shared her experience with this: https://www.deviantart.com/yuumei/journal/PSA-for-Artists-Don-t-make-the-same-mistake-I-did-577426340. I personally will take breaks to stretch my hands and massage the sore parts of my fingers. If they're still hurting after a break, then it means I'm done for the day. It's not worth risking permanent disability by pushing yourself to draw/write for a few more hours.
Check your hand/wrist alignment when you're writing. I've found it helpful to rest my wrist on a rubber/foam coaster - it adjusts the angle of my wrist just enough to relieve some of the pressure.
Explore other methods of calligraphy. I used to draw extensively in pencil (graphic and colored pencil), but over time I found that the pressure created a lot of pain in my joints, especially where I held the pencil or rested my hand/fingers against the table. I started painting instead, which doesn't require much pressure to create a visible mark (and requires much less energy to fill a space with color). Perhaps try using a brush instead of a pen from time to time, and see if that provides an outlet for you to continue doing calligraphy without wearing your hands out as much.