The center of gravity of our packs (where the heaviest items are) should be close to our bodies' center of gravity. Maybe just slightly above.

For us women, our center of gravity is quite a bit lower than that of most men. It's just below our waist, due to our larger hips. Therefore the heaviest items should be barely above waist level. They also should be closest to our backs so they don't pull away from our bodies.

Most directions for loading packs are evidently designed by/for men, whose center of gravity is well above the waist, near the shoulders.

That's why those directions aren't working for you--you are not a man! You need to put the heavier items lower, so you won't be thrown off balance. Don't pack heavier items high, as often suggested.

You may also have a pack fit problem--pack fit is almost as individual as shoe fit, so a pack that others recommend may not suit your body at all, especially when loaded. That's why we recommend buying the pack last, and taking your gear with you to the store.

I personally have found that increasingly, REI employees don't know that much about backpacking. Unless you know a long-time employee by reputation, don't rely on their expertise--most don't have much.

However, start out with the cheapest solution: pack the heavier items low, close to your waist and close to your back, and see if that makes a difference. I think it will!