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I remember seeing something like this as live-action, and I'm pretty sure it was for Goosebumps, so that narrows it down to Goosebumps book that got adapted.
The bad luck creature was sponge-like and fed on woe, so she eventually imprisoned it in a Sickeningly Sweet prison (IIRC, actually getting rid of it meant death to the victim, so she couldn't). The last scene was her picking up a potato monster.
"And as long as a sack of shit is not a good thing to be, chivalry will never die."

I'm pretty sure that this is a Goosebumps book, It was about a girl who found this creature that gave her extremely bad luck, she couldn't get rid of it, and there was something about the letter J showing up on her arm or something odd like that. At the end, when she finally got rid of the creature, another one found her, but it was like a potato and it bit her thumb.