Make sure you're using the pad of your finger.....not the joint. Pressure should be straight back from the pad of your finger to the web of your hand. Don't anticipate the shot...just concentrate on the trigger. Get some snap caps and dry fire...a LOT. Mix some dummy rounds in with your live ones when on the range to check your flinch. Just take your time and squeeze. I dont think there is a mechanical device that will fix this. Its usually mental in my experience.
I've taken shooters who flinch.....had them hold the gun on target and pressed the trigger myself while they held the gun.....bullseye. Then had them place their finger on the trigger......my finger pressed theirs....bullseye.....then let them do it alone....bullseye. It's all in the mind.
That being said....I'm a combat shooter, not a bullseye guy. If I draw quickly and put three or four shots COM within 4" thats just fine with me.