first guess would be one of the pulleys (either on the tensioner, idler, water pump, power steering pump or alternator. One of those is not kosher. This should be very visible while the car is not running. Look at each, and make sure the belt is off of the 'lip' on each pulley.

I think it was the s-types that had an issue with the idler pulley chewing up the belt, there was variations of the pulley, one had a lip, the other didn't.

Idler pulleys and tensioner pulleys (actually, the tensioner pulley is an entire assembly) are easy changes, labor wise. The others, well...lets not even venture there yet until you find the culprit.

 

This is not a problem you can fix by sitting at a computer and talking on the internet. You have a belt rubbing problem. The ONLY way to fix it is to inspect your car and find out where this is happening. This forum is great but you already know your problem and you MUST look at YOUR car!!
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Sounds to me like one of your pulleys is bad and likely running at an angle. Even a slight angle will cause the belt to flex unevenly as it passes over. The belt will go quicker than an under-inflated tire's sidewall.