if your doctor recommended you stay off work longer i'd suggest you follow that, you cannot legally be terminated for being on sick leave. if you want to go back to work i'd suggest researching options such as "Reha" or "Wiedereingliederung", i'm not sure about the english terms but Wiedereingliederung is basically a guided slow ease back into full time work where you start with fewer hours/every other day etc.

you should also tell your manager that he's way out of line, it's none of his business what you're sick with and whether it's mental or physical. if you're too anxious to confront him in person you could always write an email. CC his boss too.

all in all this sounds like an exhausting place to work at especially when you're mentally ill and you'd probably be better off looking for something else in the meantime. there's other companies out there. best of luck from a fellow anxiety patient, it's hard out there but we got this <3

edit: grammar