Another way to diagnose this is by dry firing. Watch carefully, and if you see your front blade dip left and perhaps down, it's a grip/trigger stroke/flinch issue.
I read about this years ago, and then started doing about 2x to 3x as much dry firing as live firing. Since I lived in NYC, this was about all I could do! Boy, did my shooting improve the more dry firing I did.
I had also read this advice from the legendary Buddy Brister about shotgunning, and also read top IPSC shooters do about that ratio for practice.
My shooting has improved dramatically by learning trigger control by dry firing. However, I find it's a constant thing, not a "learn and move on" type skill. When I don't shoot for a while, I revert back to old habits. Perhaps that's why the top competitive shooters keep doing it, even after a few titles.