Hi mrsmatt.
Welcome to our Mattress Forum. 
You are quite fortunate that you were able to get 10 years out of your memory foam mattress which is a reasonable lifespan for this type of mattress. Sorry to hear that you’ve started to experience pains due to foam breakdown and that the replacement mattress from the foam factory that isn’t measuring cannot be returned.
Attempting to find something that is exactly the same as your old mattress is often a frustrating and futile exercise especially if you are trying to do this on your own. There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. The best way to do this would be under the guidance of an experienced manufacturer or retailer, who has some knowledge of the previous mattress you slept on and has the experience to best approximate a mattress feel based on all the specs and your personal needs and preferences you may provide.
It looks like you are trying to find a way to make the mattress you purchased work for you. Something like this is called “mattress surgery” but you’d need to arm yourself with the knowledge, resources and understanding of how to do this to maximize your chances of success. This path is as bit as involved as the path of researching for a new mattress. Even still, there will still be some risks involved because once again you have to have enough understanding of what are your needs the interrelatedness of the layer thickness, firmness/softness, and how a layer will interact with the layers below and above… also until you actually sleep on the mattress replacing the layer(s)… you won’t know for certain whether it’s the best choice for you in terms of PPP.
The best you will hope for is to find something “in the range”, but realize that only your own careful personal testing or a detailed conversation with an experienced and knowledgeable manufacturer/retailer that has your best interests at heart will reveal whether or not the feel you are looking for is close to what your desires and need.
If you decide to go for mattress surgery … it would be well worth contacting the manufacturer and explaining what your situation is and what you are trying to do. Hopefully, they’ll be willing to assist you and offer some insights as they know their products better than anyone else. At least they can tell you the thickness and density of the memory foam layer and if it is glued to the layer underneath. Also, they can inform you about the layering of your current mattress and. I’d also ask them about the density and thickness of each layer within the mattress.
I’d be prepared to answer questions about the current mattress. Is it too soft/too firm? Is it causing pain? What type of pains? What is it you are hoping to achieve by altering the mattress? What are your specific stats in terms of body type, BMI, sleeping positions, etc.
Either way, before making the call I’d make sure to do some reading on finding out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress.
Also, the tutorial post here has all the basic information, steps, and guidelines you will need to make the best possible choices.
I’d be curious to know if they are willing to assist you…
Phoenix