Hi mville032,
I’m sorry to hear that your mattress isn’t working out as well for you as you hoped for.
Just for reference there could be other reasons for the neck and shoulder pain other than the mattress itself. Some of the suggestions in post #2 here may be helpful. An upper body/shoulder issue can connected to the pillow you are using (a new mattress will often require a new pillow as well to keep your head and neck in good alignment and it can also help with upper body pressure issues as well) and it could also be connected to the type of mattress protector or mattress pad you are using because thicker mattress protectors or pads can interfere with the ability of the mattress underneath it to contour to the shape of your body and relieve pressure points.
I would keep in mind that the Casper isn’t a latex mattress or even a latex hybrid (it only has 1.5" of latex which isn’t enough to put it in a latex hybrid category) so it wouldn’t be a good indicator about how you would do on a latex or a latex hybrid mattress. In the same way the Casper also only has 1.5" of memory foam which also isn’t enough to be a good indicator about how you would sleep on a memory foam mattress because there also isn’t enough memory foam in the mattress to call it a memory foam mattress either.
I would also keep in mind that the material or type of mattress that you tend to prefer is a preference choice and is much less important than whether the specific design of a mattress is a good “match” for you in terms of PPP. You may find that you sleep well on a particular memory foam or latex mattress for example but that other types or designs of memory foam or latex mattresses are completely unsuitable for you to sleep on so I would be cautious about ruling out an entire category of mattress based on your experiences on only one mattress (that is this case isn’t really in the category in the first place). In other words … type of materials in a mattress is a preference choice and not a “better/worse” choice and the specific design of a particular mattress is always much more important than which “category” the mattress fits in or the type of materials inside it.
In your case it appears that you don’t do particularly well with any mattress where either the comfort layer are too thick or soft or the support core is too soft to keep the heavier parts of your body (your hips/pelvis) in good alignment. Regardless of which type of mattress or materials you choose … if a mattress is “on the edge” of being too soft for you then even the foam softening that can happen in initial break in period for any new mattress can be enough to put you outside the comfort/support range that is suitable for you (see post #2 here).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”. regardless of the type of mattress or materials you tend to prefer.
Phoenix