The thing about dementors is that as long as they have “supply”
in terms of friends/ partners accepting them and enabling their behaviour, they don’t HAVE to change?
Why should they when it gets someone under their control and means they can get their own way and be indulged?
Eg take a day out with my misery guts ex friend. She wouldn’t offer any suggestions, just expect me to act like a chirpy tour guide whilst she sighed and went “no, don’t like that” (she was five years older than me). I think one day we walked past about ten restaurants on the south bank which I had to “sell”
to her before she gave her “nod of approval” to one.
If I stopped doing the chirpy act myself, she’d just sort of collapse into herself and start huffing and puffing.
What freaks me out is how my Dementors all made contact after I went nc on them, with melodramatic statements about all the “happy memories” we had together. 
It was like I’d leave a day with them feeling frazzled and tense and wound up, but they’d passed on all their anger and anxiety and misery to me? happy for THEM maybe?
It’s like their IDEA of happiness isnt “within”, it’s having power over someone else and someone else (muggins here) having to contort themselves to make them happy and entice them out of their self inflicted sulk.
So there isn’t actually any motivation to change?
The motivation for the negativity isn’t genuine internal unhappiness: it’s control and sadism. They want to feed off others and drag them down , not be content.
I know genuinely depressed people (I’ve had a mild form in the past) who focus on exercise, self-care, proper medical help, meditation,etc.
These misery gut types definitely aren’t the same.