Is it possible that you are running the belt too loose? That can cause wear. Timing belts should be pretty tight. Another thing that comes to mind is the drive pulley. You have been replacing the driven pulley, but how does the drive pulley look? If it is worn, then that may be wearing the belt out. I normally replace both pulleys at the same time, along with the belt. You don't need to change the pulleys every time you put on a new belt, but if they show any wear it is time to change them.
The other possibility is that you need a bigger lathe, maybe you are just overloading this one. Of course you need a bigger lathe.:lmao: