I had a belt drive sickle bar mower a few years back that was eating belts. It turned out to be two (2) problems. First the frame was bent slightly which mis-aligned the pulleys, we put the frame in a big press and realigned it. Next we adjusted the tension to manual specs and then adjusted a NEW idler roller to hold the belt tight on the pulleys. Problem solved.
Bet you have a similar situation. Maybe a bent pulley shaft? Idler lost?