I can't seem to hold my pen well in the correct position as show here because its too lax and the top half of the pen tends to move slightly in my hand as I write. But if I tighten my grip, then I end up like the incorrect position picture.
The reason the pen is moving around is probably because you are unaccustomed to holding it firmly (but not tightly) or you are pressing too hard, probably both. When you press too hard, the friction between paper and nib can pull the pen around in your hand. The natural reaction is to squeeze back. If the nib glides on the paper; the pen won't try to move around in your hand.
Remember it's handwriting, not finger writing. Think about moving the hand (and arm) more and letting your fingers simply act as a prop to hold the pen in the correct position. Let the hand guide the pen, not the fingers.
Get used to holding the pen firmly, but with minimum pressure, while doing something else, like reading or watching TV. When you've got that feeling and the pen stays put, then try writing. It's like the golf grip: place your hands firmly where they are supposed to be, then don't
squeeze, but don't go limp as a jellyfish, either.
Seriously, an introductory course in Tai Chi would probably help you find the correct sense of grip.
But I believe that since my life began
The most I've had is just a talent to abuse.
Hey ho, if love were all.
With apologies to Noel Coward