Timing belt

I have to guess you mean the belt is getting frayed or chewed up on one edge. That is caused by one of the idler pulleys being tipped or turned which forces the belt to one side. It is customary to replace the water pump, as you did, but also all of the tensioning devices and idler pulleys. Some engines use a stamped steel plate slid over the snout of the crankshaft to keep the belt centered on that sprocket. Be sure that didn't get left off if here was one.