a good pack trick i learned on a two week backpacking excursion is put the heaviest stuff in the middle extra clothes jackets and such in the bottom and everything else on top. also if your pack doesn’t have a lumbar strap i would suggest if you can buying or or just modifying the one you have i actually put on a waste/lumbar strap on some climbing gear this week I went and bought a 2" wide piece of nylon webbing and just one of those buckles where you push the prongs in. (did it for under five bucks) that will make tons of difference and allows you to distribute the weight differently pending on how tight it is and giving your shoulders a rest. also its nice on your bike keeps the pack from flopping around.
hopefully you can figure something out to help you enjoy the ride better. good luck!