I'd suggest changing the way you hold your pen until you don't have this problem
I've seen people suggest getting a big fat rubberized grip to put over your stylus, or wrapping your pen in "tourna grip" tennis grip tape
But in the end you just need to find a stable way to hold your pen: Experiment with different ways of holding it until you find a stable one, it's not going to look the same as a normal writing grip
You can look at play videos of some of the top players to see the weird grips they use, but it's going to come down to what's comfortable for you
Also, make sure you keep your pen + tablet surface clean of skin oil, use a little bit of dish soap + water on a towel and be careful not to get the electronics wet