Cc: My pack does have hip & sternum straps which I cinch up as tight as possible which feels good & snug. But when I have the hip straps fastened so the pack is sitting on my hips, the shoulder straps extend up above my actual shoulders and rub against my neck. It feels fine when I first put it on and all the weight (12-15#) seems to be sitting on my hips, but then 5-6 miles later... ouchie!
Your pack just doesn't fit properly. If you continue to use that pack, the problem will get worse. Below is a post I made earlier about pack sizing and proper fit. I hope it may give you some insights that will help.
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As to fitting a backpack, the
volume or carrying capacity of a pack is not the same thing as that pack's
size. Size in a backpack is not based on the "volume" that the pack can carry, it is based on the "length" of the frame. It is also about whether the shoulder harness feels good, because the actual shape of the shoulder straps determine if they will be comfortable with your body shape.
It is vital, for a comfortable fit and so that the pack can be properly adjusted, to have the proper measurements made of one's spine length. Hipbelt sizing is pretty straightforward, and many quality packs now have the ability to swap out different sized hip belts.
It is actually quite easy to take the necessary measurements. Having someone help for a couple of minutes makes it a snap. Thousands of folks have successfully purchased packs online, and almost all of the online backpack retailers have easy exchange or return policies should that be necessary. I would also point out that purchasing from a brick-n-mortar store is no guarantee of a proper fit or of comfort. I have had numerous experiences with people who were given 'expert' advice by a store employee, only to come to me for help as to why their pack didn't feel really comfortable or 'right'. A trip to watch the REI returns counter will underscore that point.
Some manufacturers offer choices of an "S" strap shape or a "J" shape. The names reflect the actual shape of the strap. The "J" strap is the traditional shape. As women became a bigger part of the backpacking market, one of the most common complaints was that the "J" strap would rub and press against the bosom, which is not a problem for most men. The "S" shape helps eliminate, or at least markedly reduces, that issue. When fitting and trying out a pack, you can see how this might be an important thing to pay attention to. Not too surprising, there are men who prefer the "S" shape as well because of having a larger chest size. Some manufacturers have combined and modified the two shapes into a sort of 'hybrid'.
Here are a couple of videos that may help with the above information. Ignore the product references; the information applies no matter what pack is being considered.