My cousin did the same thing with guitar and piano for like six months before he just picked guitar and stuck with it. I've noticed the people who flip around are usually the ones who care about picking right, which is a good sign, but the switching itself is what slows you down. That file renamer you built is worth more than any tutorial, so keep going with Python and stop second guessing it.

How big are your squares exactly, because 6 inches sounds right for a door but way too coarse for a crease near a body line where the metal gets tight and you can't just bump it out with a spoon. Are you marking the dents before or after you strip the paint, since a grease pencil over filler and primer can smear and then you're guessing where the low spots actually are. And when you check a square off, are you going by feel with your hand or are you hitting it with a straight edge every time, because one pass with a file board will lie to you on a crowned panel. The part I can't figure out is how you keep the numbers from rubbing off once you start shrinks and hammer welding, do you re-mark as you go or just trust your memory on the grid layout? Two hours on one quarter is not slow, that's pretty normal when you're chasing oil canning back and forth, the grid just stops you from undoing your own work.