Yeah, it does sound as if you might have killed the padding. Cascade Designs Therm-a-Rest pads and their ilk are pretty tough, but if you compress the foam for long periods of time, theyll lose their memory" and wont re-inflate well. The Therm-a-Rest people recommend that you store the pad flat and with the valve open, somewhere such as under a bed or perhaps upright in a closet. You also want to keep it in a fairly dry place. If you dont have a place where flat storage is possible, Ive had good luck storing a pad loosely tied into a circle with a bungee or other strap.
So, what to do now? Replacement might be the best option. The Trail Lite Therm-a-Rest ($50; thermarest.com) offers the best range of comfort in a light-weight, ideal-for-backpacking package. If car camping or if weight is less of an issue, the Trail Comfort model ($75) is cushier but of course a bit heavier.
Of course, these days the Therm-a-Rest people have plenty of competition. Big Agnes has done well with its REM Two-Track pad ($75; bigagnes.com), a fairly thick pad with nice features such as a nearly unbreakable brass valve. REI is in the sleeping pad business as well. Take a look at the REI Lite-Core 1.5 ($75; rei.com), which is tapered to better fit a mummy-style bag. Care will be the same for any self-inflating padstore them with the valve open and as flat and un-compressed as possible.
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