Putting some pressure on the left hand side of the grip will also help to stabilize the muzzle from being pushed left.
Holding the thumb straight out toward the target helps to push the thumb section the hand forward to apply some pressure on that part of the grip.
But every grip is different and not all people grip the gun the same.
Another idea is to squeeze the thumb inward in an effort to try to minimize any lateral barrel movement.
But if it's an eye issue than I'm off base.
Maybe you could use a single shooting lens that's adjusted for your arm's length when your holding the pistol.
Some folks have them made by eye doctors who let them bring a pistol or rifle into the office to try out different lenses. Even if it's an air gun with a similar style of sight mounted on it. Some doctors specialize in shooting glasses for the various types of guns, and many have an office in their home.
I hope that you're able to figure it out.