I suspect this may not be the answer you were hoping for, but here it is.
Based on your question, I am going to assume that you are still learning some techniques and that you have not yet reached a level of advanced proficiency at the guitar, and that you are not an otherwise advanced player who is struggling with one specific technique. Please let me know if I'm wrong.
When you master picking, your hand will be relaxed as you see in this video. The tension and pain are from trying to push your body to do something it has not yet mastered. As you master this technique, it will become easier and easier and you will expend less and less energy. And yet the advice to "try less hard" is not very helpful for you right now. But what you can expect as you get these techniques down is that mastery will be marked by ease and relaxation.
You may not be able to "learn" this technique in the sense of using disciplined practice over a short period of time to acquire it....