What I would need to know to make a more informed recommendation would be your height, build, and approximate weight. No, I / we are not being judgmental, but the solution might be simple. These base factors are key to making an informed recommendation.
Let me explain.
You mentioned the shoulder straps were "cutting" into your armpits. That could be a sign that the store clerk was trying to fit you into a Small/Medium sized harness / pack panel, instead of a Medium / Tall version of the same bag. Your torso length makes a difference. The better outdoor stores have plastic fitting frames for the better brand rucksack s that can help identify what size rucksack you need.
The difference is several inches vertically. With the proper bag size, and harness adjustment, the straps should not come anywhere near your armpits. If they measure you and, like me, you are at the cusp of going from medium to large, try BOTH size harnesses.
In the Osprey product line a typical rucksack capacity in a Medium / Large frame will be two liters LARGER than one in the Small / Medium harness size. For example, my Osprey Kestrel 48 liter bag, becomes a 46 liter bag if I had the smaller harness. However, it is still advertised as a 48 liter bag.
Also, and as others have commented, dead weight in the form of sandbags or rice, kitty litter, etc. rides differently than soft clothing you pack. I do not understand the physics. It just DOES.
If the rucksack waist belt is correctly located over your hips, and the shoulder harness correctly cinched so the top of the rucksack rides high and close to your shoulders (minimal air gap a the top), the rucksack SHOULD ride in parallel with the motion of your customary stride. The rucksack must be parallel to your spine, not at an angle. The pressure on both shoulders should be the same, as it should be for your hips.
If you waddle or wiggle normally, so too will your rucksack. This is normal. However, once the bag is correctly fitted and adjusted to YOU, the apparent motion should be no more pronounced than the rest of you. Remember, everyone is different. Your experience, fitting, and results with a certain rucksack will necessarily be different that others. We are all, literally, the sum of our pieces. No two skeletons are identical.
True too, the weight of the loaded rucksack is SUPPOSED to ride on your Iliac Crest, the large hoop bones of your hips. Now, if you were blessed with larger than usual Iliac Crests, your load will be relatively lighter to bear as you have a better quality built-in "shelf." I have a similar situation. I inherited my Sicilian grandmother's hips. So, my Osprey hip belt rides perfectly.
Also true, as others have said, there are other quality brands out there. Keep trying. Also, do not limit yourself to just "women's" rucksacks. Try everything. Buy what works best for you. Depending on your personal physical statistics, you might find than a rucksack designed for a man might just fit you better. I try women's rucksacks. Color aside, several Deuter brand rucksack come close to a perfect fit for me. You might try them, as others have mentioned.
As you do this, please keep in mind that the TWO MOST CRITICAL BITS OF GEAR you will invest in are your boots / shoes and your rucksack, in that order. You will wear them every day for a month or more. They will become part of you. They must fit you perfectly and be fully suitable to purpose. Shop accordingly.
When I remove my rucksack at the end of a day's walk, I feel naked. My Osprey Kestrel 48 is so well adjusted that it fits like a glove. It feels comfortable on my back. Yes, it can get hot, sweaty, and wet back there, but it is never uncomfortable or painful.
I hope this helps.
