Hello. I am having constant problems wit...

Hello. I am having constant problems with my 1997 chevy cavalier serpentine belt. My old wrecked belt was working fine but when it started wearing too thin i got a new one. As soon as the new one was put in it starting squealing very loudly when i accelerated. My mechanic used sandpaper to wear down the edges of the belt a bit and that worked at first but the loud squealing returned. This time i used sandpaper to eliminate the noise but i guess i used it too much because the belt soon shredded while i was driving { almost starting a fire}. I purchased another new serpentine belt..and this one worked better but still squealed a bit...so i thought i would live with it.
Not long after my air conditioner started going so i bought an ac bypass pulley and had that installed recently...and boy the noise has returned with a vengeance. The belt is emitting a screaming squeal whenever i accelerate now....and i know the ac bypass pulley is meant to be used with the same belt...so now iam lost. Maybe i have the wrong pulley but i checked that out and it is the correct one. Now im thinking of getting a new belt again. My mechanic is a head mechanic at a dealership { a good friend} and he thought the pulley wasn't right for the vehicle { soemthing about the bracket not fitting properly.} so i don't know who to believe. Any suggestions? HELLPPPP!