I have isolated 2 problems and solved
the overall issues.
1) I have been sitting on the starboard tube, using my right hand to hold on and left to steer/throttle. Due to the inclination of the hull (bow up) this compressed my already crooked back in the wrong direction, forcing it to be even more crooked while making way in the dingy, which pinched my nerves. This is why "just slow down" actually made things worse! I was sitting in that horrible nerve pinching position for 3x longer, which was agony compared to getting it over with. Solution:
I will now sit to port, steering/throttle with my right hand. Too bad the tiller is on the wrong side.
This will curve my back the better way, actually helping straighten it during dinghy rides.
2) Weak core. Still working on this one. Continuing to try to straighten out the back by evening out the muscles on both sides.