In reply to afx22:
Contrary to the myth jamming is LOTS of technique in addition to pain resistance. Hand jams are probably the easiest to learn and not normally painful, so I assume your technique is flawed somehow.
I would recommend to get to the bottom of some crack, maybe even on a building*, and simply practise the first two or three hand placements, initially not even leaving the ground.
One common beginner's mistake that I could imagine may cause your problems is to remain too central in front of the crack. In this case, all the jamming force must come from expanding your hand and/or from actively rotaing your forearm with pure muscle power in a not very ergonomic posture.
Getting your body weight slightly to the side of the hand you have just placed will recruit your body weight and lock in your jam by "torqueing" your forearm once your body start to sag a bit. This works both thumbs up and thumbs down.
CB
* For the last three years I have had two square concrete pillars in my office which make for a 3.5m high, absolutely smooth offwidth (14.5 cm wide). Just locking in and loading the jam every now and then (and occasionally climbing up to the ceiling) has done wonders for my hand / fist stacks and calf bars! Who knew that 1970s neobrutalist concrete architecture can actually be good for something!