I taught LE firearms for years, coached in the Corps, and taught private classes. Two suggestions.
1. Someone else hit on what I though of first.
You may not be flinching, there are two things you may be doing that can cause those low shots.
a. Milking the grip: squeezing the trigger with your whole hand rather than the trigger finger alone.... aka milking.
When you tighten the bottom three fingers at the last instant as the trigger breaks, it pulls the muzzle down and the shot low. YOU control the trigger with the trigger finger, you grip the gun with your hand. Concentrate on TRIGGER control.
b. Looking Over the sights at the last second to see the impact of the shot.
When you do this, you drop the sight to see the target. I laugh and tell my students that the HOLES don't leave or hide, they'll still be there after follow thru. Make an effort to watch the front sight all the way thru the shot. Think FRONT SIGHT and focus solely on the sight, see the sight clearly as you apply trigger pressure until the shot breaks as a surprise. The last thing you saw when the shot broke should have been the CLEAR TIP OF THE FRONT SIGHT.
A drill to help you with trigger control is very simple and very cheap. All you need is a spent casing from your Glock 19... that's the gun you mentioned, right? (regardless of the gun used, I normally use a .38 casing)
UNLOAD and physically confirm a safe weapon!
Rack the slide to cock the striker
Balance the empty casing on the end of the slide just in front of the front sight.
You won't be able to see the front sight.... no problem
(You may find the drill easier if you paint a vertical line on the casing with a Sharpie)
Extend the pistol toward your target and sight as if the center of the casing was your front sight.
Squeeze the trigger smoothly concentrating on the sights, using the casing as your front sight ... until the trigger breaks.
If your trigger pull is smooth and undisturbed, the case will NOT fall off the slide. In fact, it won't move. It will flinch slightly when the striker breaks. Nothing else.
Tilt the glock, catch the casing and repeat the drill. Recock the striker by drawing the slide to the rear to cock the trigger / striker.