Thumb pressure is unwanted.
Re-grip the pistol. (let yr non-shooting hand give the pistol into yr shootinghand).
The pistol is held in position by the 2 middle fingers. Pinky and thumb do not apply any pressure, nor do the finger tips.(they do touch the grip).
To check, aim, close yr eyes than re-open.
Pressure: do not overtighten grip pressure (white fingers). You can fiddle at bit with this. Most use the trick to over-grip than loosen.
Regarding grip fit as stated above:the middle part of yr fingers should be straight on the foreside of the grip. If the grip is too small they wont lay on straight.
Fingers have to get across the grip fully otherwise its too big.