Wow, I could have written the same thing. I'm average height/weight as well, and I've been trying to find a mattress that doesn't sag within a couple months. I'm normally a lurker on most online forums, and I was joking with my wife that my back pain and sleepless nights have gotten so bad that it actually got me to create an account here.
In my case, I've tried a Tempurpedic Deluxe which only lasted maybe 6 months and turned out to sag a huge amount and sleep really hot. We also tried a Keetsa Pillow Plus that was awesome (nice and firm and no pain!) for about 2 months and then started sagging, but not enough to allow for a warranty exchange. And of course, I waited longer than 90 days to return the mattress, so I'm SOL on that one.
Growing up, it seemed like mattresses lasted for years. Now it seems like a stretch to get one that lasts longer than 6 months.
I'm thinking about the Sleep Science hybrid mattress currently being sold by Costco, which at least is returnable if it doesn't work out. But otherwise, I'm pretty much lost at this point.