Hello outdoors people,

I have done several short backpacking trips recently (spring through fall of 2014) and my pack has served me well as far as holding my stuff and functioning well. However, I need advice on how to alleviate the pain in my trapezius and upper back after hiking with my pack on. I can go 10 miles or more but by that point my traps and upper back feel horrendous. I have an REI Flash50 (50 liter pack). I am an athletically built guy but my waist size is about 29" and even when I pull the waist straps to their very tightest, I would still like to tighten it another inch or so. When hiking with this pack, I tend to jut my head forward to offset the pull of the pack on my back. I do this unconsciously and am constantly trying to pull my head back and walk with a straight up and down posture. Any advice? Do I have the wrong pack? Am I too thin? Is the only solution to lighten my load? I'm not sure how much it weighs when full but I'm guessing 50lbs (?) I could probably do some things to significantly lighten my pack, but I'm curious what advice people might have.