"I hold the back of the neck with my thumb but I’m applying too much pressure which causes my thumb to hurt." That nearly always means that the guitar is set up badly. Take the guitar to a luthier or a good guitar technician to have the neck relief adjusted, the string heights at the nut set, and finally the string heights at the bridge set. The adjustments must be done in that order.
It's possible that you simply have a weak fretting hand. Exercising with a wrist roller, and a gradually increasing weight, will increase your grip strength.
Hold a slightly soft sponge ball, sized midway between a golf ball and a tennis ball, between the pad of the thumb and the middle joint or pad of the middle finger. Close the other fingers to stabilize the ball, then squeeze hard with the thumb and fingers, and hold for 30-60 seconds (if you can't, ease your grip a little; tighten your grip as your strength improves).
Perform the roller and ball exercises on alternate days.