Hi melknee,
Unfortunately your experience is very common for consumers that purchase major brand mattresses which tend to use lower quality and less durable materials (in the upper comfort layers especially which are generally the weakest link of a mattress) and which are much more likely to soften and break down than mattresses that use higher quality and more durable materials (see the guidelines here).
The first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and more importantly know how and why to avoid the worst ones (including major brands or any mattress that uses lower quality and less durable materials or where you aren’t able to find out the quality and durability of the materials inside it).
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
I don’t make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or materials because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components would be the best “match” for you in terms of “comfort” or PPP or how a mattress will “feel” to you based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I can certainly help you to narrow down your options, help you focus on better quality/value choices that are available to you either locally or online, help you identify any lower quality materials or weak links in a mattress you may be considering, act as a fact check, answer any specific questions you may have along the way that I am able to help with, and help with “how” to choose but only you can decide which specific mattress is the best match for you based on all the parts of your personal value equation that are most important to you.
The choice between different materials or components or different types of mattresses is a preference choice and not a “better/worse” choice (as long as they all use good quality and durable versions of the materials you prefer) so it’s always a good idea to do some local testing to find out which types of mattresses you tend to prefer (see this article).
I would also be very cautious about brand shopping because you are buying a specific mattress not the brand and all manufacturers have access to the same or similar components and materials. Most of the more heavily advertised and most commonly available brands that you would recognize are the ones I would avoid anyway and many of the better manufacturers are smaller and are only available locally or regionally (or online). The name of the manufacturer on the label also won’t tell you anything about whether a specific mattress would be a suitable choice for you in terms of PPP or whether there are any lower quality materials or weak links in the design that would affect the durability and useful life of the mattress. There is more about the risks of brand shopping in post #5 here and post #12 here. Outside of careful testing for PPP (hopefully using the testing guidelines in the tutorial) or a more detailed conversation with a manufacturer or retailer about whether a mattress would be suitable for you if you can’t test a mattress in person … I would focus much more on the type and quality/durability of the materials inside a mattress (which are important to know anyway) than I would on the name of the manufacturer on the label.
The tutorial post includes several links to lists of the better online options I’m aware of (in the optional online step) that includes a very wide range of mattresses in a wide range of prices as well.
If you let me know your city or zip code I’d also be happy to let you know about the better options or possibilities I’m aware of in your area as well.
Phoenix