So, try this: A tump line. Get some nylon webbing, and attach it to the top of each shoulder strap. Adjust the webbing so it runs over the top of your head, from shoulder to shoulder. The idea is that you want to transfer as much weight as possible to your head and neck, getting it off your back and shoulders. Keep the shoulder straps on the pack, but use them for balance, not load-carrying. It'll take some getting used to, but it works, absolutely. People in most cultures, in fact, carry loads in this manneronly Westerners have taken the backpack route.
Give it a shot, then tell me how it worked. I think you'll be pleasantly surprised.