Kristal! So now you are a slinger!
Excellent!!
I have come to believe that we were born to sling. That is, so many human games and hobbies are of a "ballistic" nature. What is a child's ball? Of course, it is a PROJECTILE. Indeed, I remember reading somewhere that there are some paleoanthropologists who believe that early protohumans gained a great survival advantage by being able to throw rocks at animals for food. This initiated a positive feedback loop where those who could throw better survived longer to have more offspring who would be even better throwers, etc......
I, too had some difficulty with my first slinging attempts, but you will get better. Just don't get arrogant or cocky. Your novel's character has special power, but you are a mere mortal like the rest of us.
It took me about 100 to 150 throws to get to the point where I could get a good trajectory on the stone. Another 100 throws or so before my slinging became really positive and reliable. I am now at 350 to 400 and can hit a tree trunk from about 20 paces about 70% of the time if I CONCENTRATE. ( I use the Apache technique. )
Don't push yourself too hard. Give yourself the time. everyone learns at their own pace.
My suggestion is to not try for accuracy at first, but rather to go for good form and to try for, what I like to call "slinging for trajectory,"
that is, I try to get the stone to fly in a nice, smooth, "elegant" arc ( aimed only in a general direction. )
It gives me great pleasure when I achieve this. As I said, I suspect we humans have an instinct for this! And I congratulate you; you are now a member of a sort of "club" that is at least 9,000 years old!! .......Dan