Ford escort serpentine belt keeps coming off.when it rains?

I have a 98 escort zx2 every time it rains my serpentine belt comes off. I dont think its the tensioner because its so hard to pull when i put it back on. Ive replaced the belt 3 times now due to it sheredding. Any other ideas? This is getting really annoying and getting another car isn't an option right now

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