First, welcome to the forums.
With regard to the pack, I think you’ll probably have a near-impossible task trying to make it work. (The only thing I can think of that is as important as a properly-fitting pack is properly fitting shoes or boots.)
Are you a medium in Osprey packs, or a medium in another brand? (“Medium” can be different for different brands.) However, before you give up, do two things:
1. Load your typical load of gear (including food and water) into the pack, as you would for a weekend trip, and try it on; play with the various adjustments on the suspension, to see if, by chance, it will work.
2. Since it’s loaded anyhow, take it to the shop where your friend bought it (if you can; if you can’t, take it to REI - they’re good about helping), and let them take a shot at making it fit you correctly.
Then, if neither of those things work (they probably won’t), start looking for a new pack. You might be able to sell the one you have, or at worst use it to help someone else get on the trail.