I'm not questioning your honesty but I think this needs to be said for consideration. The story you tell and the experience you describe simply is not consistent with the the vast majority of the literally hundreds of millions people that sleep on quality mattresses in the United States.
My weight varies between 225 and 235. I sleep on a XL twin so that means I pretty much sleep exactly in the same place. I've never had a problem with a mattress, even economy quality ones and I've never had a problem with quality memory foam toppers that are at least 4 lb. density. The topper does tend to wear out after about three or four years.
I would suggest you get at least a double or perhaps a queen size bed. Since you say you have had problems with memory foam toppers and giving you the benefit of the doubt as far as assuming you understand the differences in quality and density of memory foam I suggest that instead of memory foam you get a latex topper.
Here is a description from one particular web site that I provide just for reference and not as an endorsement of the company but as a reliable informative link.
Latex toppers and memory foam toppers, (quality ones) have pros and cons when compared to each other. One pro for the Latex topper is support and durability while still providing cushion. Resilient cushioning.--
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