Spent 6 hours chasing a 1/16 inch gap on a face frame before I realized the problem was the wall
Hung a 42 inch upper cabinet in a 1920s bungalow in Cleveland last Tuesday, shimmed and re-shimmed the box four times trying to close a gap on the left stile, and only after my helper held a level against the plaster did I see the wall was bowed almost half an inch over 3 feet, so who else has given up on the cabinet and started fixing the wall instead?
1920s bungalow in Cleveland is basically the final boss of "nothing is square." I bet that wall has been bowed since before your grandparents were born and it's just been sitting there laughing at every level you own. Six hours on one gap is rough but honestly the box probably was fine the whole time, it was the house messing with you. At some point you just gotta admit you don't own a cabinet problem, you own a plaster problem. Good luck with the wall, that's a whole different kind of rabbit hole.