Pelican Parts Forums - How does one go about picking the right mattress?

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11 years on our Sleep Number (Select Comfort) mattress. Love it. No leaks.

I chose... poorly.
The girlfriend and I convinced ourselves we needed a memory foam mattress, but she didn't want something too firm. We ended up lying on a whole bunch of them and settled on a Tempurpedic Cloud Supreme Breeze, which is their 3rd softest out of 8 firmness levels in the Tempur line. I'm convinced it's too soft, because I end up sleeping on my stomach, and I wake up with a sore lower back every morning. Last night I got up and slept in the guest room on a conventional mattress, and this morning I felt great. We've had the Tempurpedic too long to send back, so now I don't know what to do. Move to the guest room? Buy a firmer foam? Go back to a conventional? One reason we wanted the foam is she gets up at 5:00 for work and I don't get up until later and she was waking me up with the old mattress.