dunno if this will help, as IANAM (i am not a messenger) but i try to keep the center of mass of my bag centered between my shoulderblades, and the strap just tight enough to keep the bag there. (not always workable if i have large load, but it's usually just a laptop some books, water and some tools).

is the discomfort from the strap or the load itself? if it's from the strap, does the strap have a pad, and if so, does the pad go up and over your shoulder?

if it's not the strap, you might want to consider starting w/ a lighter load to let your body adapt a bit before attempting the 'human mule' bit.