my daughter's otherwise very gentle and scaredy cat claws them to bits when she has to go in her travel cage.
Rosie goes in with only a few very naughty words snarled at me, no claws, and isn't frightened in the car, it's still her territory. Paddy isn't worried either.
Rosie just
hates new territory and gets frightened very easily - she doesn't go beyond our small garden most of the time and if you shut the door she wants to come in instantly - her escape route has to be there and she's in and out and in and out like a yoyo checking. She's also very very very stubborn! I'm sure she'd do a repeat performance of not eating (for over 2 days) if we leave her again
Paddy has a much bigger range and is extremely confident outside - well anywhere! I'm more used to scaredy cats and it constantly amazes me.
I think you have to start when they are kittens - but then they'll still make up their own minds what is acceptable and what isn't once they grow up. So Possum may still have felt the same even if you'd started when he was little
As my parents played with Rosie for hours when she was little, fed her cream and expensive treats, and stayed when we went away last year, you'd think she'd love them to bits and feel happy there - but no, not my stubborn little madame.