If you feel tension in your hand holding a flat pick you're not holding it properly. You should be able to keep it up for hours with no ill effects (other than from the beers which is a different issue...)
I think of the old fencing master's advice: it's like a little bird. Hold it too tightly and it dies, hold it too loosely and it flies away. -Glenn
PS Betsy, glad you've made some progress with the condition.