I do not disagree with any of the advice you have been given. I will only add that I recently discovered what had been causing me to shoot nice groups to the left of my target. (I have been using some very light limbs to work on my form and I have been paying particular attention to my shot process.)
For me, it was the elbow of my bow arm. I was getting sloppy. When I made a deliberate effort to rotate the elbow of my bow arm until the elbow was pointed "out", parallel to the ground, the tendency to group left went away,immediately.
If my elbow was even slightly "pointed down", even a little, my group started shifting left. Too much, "pointed down", of course, caused the bowstring to whack my arm guard every once in awhile. Even a little slop in my elbow position can cause my arrows to shift left, 2"-4", at twenty yards, for this right handed archer....
Good luck.