The only time I toss a belt is when it is totally glazed over. I've had and still do have belts that're 8 to 10 yrs old and a couple 3 in my trailer which drops below -0- in the winter and well over 110 in the summer if it's been closed all day, (it's black). It's dry inside if its raining but the moisture content is high, condensate all over metal tools on cool nights and mornings. I used 2 6 or 7 yr old belts from the trailer late this spring and they were fine.
I can't say whether the belts are high quality or not I've had them so long and bought them at the Lumber yd.
Is it possible you put the belt on backwards? The belts have a rotation direction arrow.
Work smart not hard!
Never bite the hand that looks dirty