Everything said here is very black & white but I agree it’s time for very tough love. Who is the parent here?
There clearly might be a back story or indeed you’ve allowed her for two decades to rule your roost?
I’d put anything she leaves lying around, on a daily basis, into a cardboard box and shove it in her room. She can pick out what she needs/wants from that. Her own personal mess box!
Id clear up the messiest communal room like for example your living room, dust hoover & put stuff away and tell her that’s how you want it kept from now on. No dirty mugs or glasses, clothing, slippers, chocolate wrappers, crisp packets, paperwork, absolutely nothing to be left lying around. Stick to it. Your new discipline.
A month isn’t long to get her ass into gear to find a job and/or move out. In your shoes I’d say she’s got to be employed and out by (say) Easter?
Thence, tough love Mum. You change the locks and don’t share the key.
I left home at 16 to work abroad! You can do this mum. You can do this. Go for it.