Hi Kitchz,
While it’s not as common for back sleepers to have pressure issues with a mattress that has about 3" of memory foam in the comfort layers … based on your description it sounds like your mattress is a little too firm and that the comfort layers may be a little on the thin side for you.
The most likely solution if you are looking to improve pressure relief and soften up your mattress would be to add a topper.
If you are confident that all your mattress needs is a little extra softness and that your “symptoms” are from pressure points or a comfort layer that is too thin for you then post #3 here and the topper guidelines it links to should be helpful. If you prefer the “feel” and response of memory foam over other foam types and since you are in a lighter weight range I would probably consider a softer memory foam topper in the 4 lb range. “Just enough” in terms of thickness is usually less risky so you don’t exchange pressure issues for alignment issues and I wouldn’t go any thicker than a 2" topper. Even 1" could make a difference because back sleepers generally need thinner comfort layers to reduce the risk of their heavier pelvis sinking down too far and causing alignment and lower back issues.
Phoenix