To take a different tack: if you're pinching the arrow you might have a problem with
a) deformed hands, with fingers that all end up overlapping the centre of your palm when you make a fist
b) a habitually poor finger position before you draw
c) a bow too short for your draw, so that the string pushes your fingers together
d) a poor elbow position, so that your fingers aren't in line with the forearm & the direction of pull from your finger flexor tendons moves the fingers sideways onto the nock
e) something else that someone will point out that I forgot
Time to yell "coach!", or video yourself shooting.