Hi rd11,
If your mattress is either 11" or 13" thick then it would have more than one layer and probably a combination of ILD’s in its construction. There could also be other materials in the mattress (the law label will tell you which materials are in your mattress but not the layers or the type of each material).
The whole topic of ILD can also be very complex because ILD itself is only one of many factors that determines the softness and feel of a mattress and is only really meaningful if you are comparing the same type of latex or material (either Talalay or Dunlop) in the same layer thickness. Post #6 here talks about the difficulties involved in using just ILD to compare materials and post #2 here outlines some of the specs besides just ILD that can affect how foam performs and how layers interact.
Because every layer interacts with every other layer in many ways … it would be helpful to know the details of the layers in your mattress because this can make a difference in what direction you choose to go but it sounds to me like you may need a little bit more thickness/softness in the top layer of your mattress to improve your pressure relief. Lighter weights generally need a little softer foam than people who are heavier (or who have their weight more concentrated in certain areas) because they will sink in a little less and foam will feel firmer than it would for someone who was heavier.
Post #2 here also has some insights into which types of changes may have an effect on different “symptoms” you are experiencing on a mattress.
Even if you don’t know the specifics of the layers though (or can’t find them out) then if you are looking at pressure issues (which your symptoms certainly seem to indicate) then adding a topper may be very helpful and post #2 here (and especially the posts it links to as well) will provide some good guidelines for choosing a topper that can improve your “symptoms”.
With pressure relief … it’s usually best to have “just enough” in terms of thickness and softness in your most pressure prone position (for most people on their side) so that the thicker layers of soft foam have less effect on your support / alignment on the mattress in your less pressure prone sleeping positions (back and especially stomach).
I would also keep in mind that there is a break in period where your mattress will go through some initial softening as foams lose their “false loft” and cover materials stretch to different degrees and there is also an adjustment period to any new sleeping surface with different properties than you are used to and your body “unlearns” it’s old sleeping habits and becomes more accustomed to the new sleeping surface. Both of these can take up to 90 days or so but in most cases it’s less than 30. Making changes too quickly before either of these processes has happened can lead to the need to make further changes when the mattress has completed it’s initial break in period or your body has become more accustomed to a new sleeping surface.
Phoenix