Hi franmary.
Welcome to our Mattress Forum! 
I am sorry to hear that you had so much trouble in finding a suitable mattress and that you are experiencing pain.
iRest is a Kingsdown mattress, and they tend to use lower density foams in their product.
The major brands all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors The componentry used in their mattresses tends to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here. along with post #3 here. and post #12 here. and post #404 here.).
As you’ve received poor advice from the same store three times, you may wish to avoid that business for your future mattress needs.
I would focus on assessing what your needs and preferences are for your particular situation and on finding a few knowledgeable and transparent retailers or manufacturers (either locally or online) that sell the types of mattresses that you are most interested in that are in a budget range you are comfortable with and that you have confirmed will provide you with the all the information you need about the materials and components inside the mattresses they sell so you will be able to make informed choices and meaningful comparisons between mattresses and then carefully locally testing mattresses that use good quality components.
The best way for you to evaluate a mattress or how certain componentry might perform for you would be to find 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 before making any purchase.
I am not very sure what you’ve read since you found the site but the first place to start your research is the mattress shopping tutorial here (there is a condensed version of it at the end of that post) which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones. 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”, firmness, and PPP 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 he will sleep), durability (how long he 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 (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Because of your higher BMI finding a suitable mattress presents special challenges and generally requires firmer materials (in the support layers especially). This could be firmer latex or innersprings (the type of support component would be a personal preference and in the right design either could be suitable) or even a zoned construction. The same overall guidelines apply with higher weights though that PPP (Posture and alignment, Pressure relief, and Personal preferences) along with using high quality durable materials that will maintain their feel and performance for longer periods of time are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter and because no materials will last as long with much higher weights the quality and durability of the materials and components is even more important than normal. I wouldn’t “rule out” any types of mattress and base your choices on your own personal testing.Post #3 here~ has more information and suggestions about heavier weights that is worth reading.
There are also a number of forum posts and threads about mattresses and higher weights and a forum title search on heavy (you can just click this) will bring up many of them. Some examples include post #2 here and post #2 here and this thread
Once you had a chance to read through this information and decide what type of materials would best suit you and perhaps narrow down a few mattresses that use good quality and durable materials, if you have more specific questions I would be happy to assist you.
Phoenix