one thing to check is make sure insides of the pulleys are smooth looks like your pto and mule drive pulleys look good one on the deck I cant see from your pics the other thing is if you center bearing is going out it will let the pulley come forward and run the belt on a angle and wear the belt and what number belt you running on your mule drive should be a 3003a if I remember right

The pulley that does NOT spin freely. If there is resistance, noisy or wobble, time for repairs.

When my long deck drive belt failed a couple of years ago it turned out to be the spring loaded idler that was bad. If there bearings in it do not allow it to flex as it need too then it will cause rubbing and the belt will fray and then fail.:beerchug: