At this point your rubber seal is "welded" for lack of a better term, to your motor case.
By your description I assume the filter is now somewhat shredded? If so, remove what is left of the can and expose the baseplate. Use a couple screws through a steel filter tool into the baseplate. Make sure they are short and thick. Then you can twist it off. I suggest turning it tighter slightly if possibe first. Heat might help as well.