David,
Not sure if I can be of much help to you as my knowledge of progressive supranuclear palsy is limited.
That's a good thing, your positive response to levodopa and improved mobility!
Drs can be wrong in their initial diagnosis and also in the rate of disease progression over what can often be an extended period.
Hang in there and perhaps try and take it one day at a time.
Easier said than done but worth a try!.
Regards.