I have a shower head that leaks pretty bad when it is turned off. I purchased a replacement cartridge and have been looking online for how to replace the part. I can take everything apart OK, except it seems like there is some O-ring that needs to be removed to allow the old cartridge to come out. There is a lot of buildup on the part and it seems like the O-ring may be rusted on. I've attached a picture, tried spraying on some CLR and WD40, but when I get a wrench to try to twist off the O-ring, the whole contraption starts moving. I definitely don't want to break anything on the inside of the wall. Is there a good way to get this O-ring off, if that is even what needs to be done to complete the cartridge replacement?

Thanks for any help!! Hoping to not have to call a plumber...

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