I know I've written about this in terms of boots, but makers of backpacks have made incredible strides in backpack comfort. I'm on my third backpack (forth if you count the external frame one) and I'm worn out pretty much none of them. What I have learned over time is that they are in order of decreasing discomfort to use the double negative that I'm kinda fond of.
Not only that, these days backpack manufacturers have created a sizing system so that you now have the equivalent of a shoe size. Now I can walk into any outdoors store and ask about backpacks that fit a size 15.5 (inches I believe). If your equipment is more than 2 or 3 years old, seriously consider taking a closer look at the new stuff. Backpacking gear is not that expensive, and your body will thank you.
You also owe it to yourself to have your pack fitted to you by a knowledgeable person which should mean an employee of a reputable outdoors store and not a Significant Other (hearkening back to Snowboarding: Domestic Discord). You do not want your S.O. to be blamed for any soreness. There are a lot of fitting details and variables and getting it right can make a real difference in pain free or real pain.