Hi langanke,
I wish I could have given you better news but 8 years is actually a longer useful life than many people would have with a major brand mattress and there are many examples on the forum of people who have only been able to sleep well on some fairly costly major brand mattresses (including Stearns & Foster) for about 3 - 5 years.
You’ve probably read this but just in case you haven’t … the first place I would start your mattress search is the mattress shopping tutorial here which has all the basic information, steps, and guidelines you will need to make the best possible choice … and know how and why to avoid the worst ones this time around.
Phoenix