You gave her quality of life as best you could, and she enjoyed her life with you. Please don't blame yourself. Personally I really don't like the idea of 'indoor' cats - I think it's awful to keep them permanently cooped up inside.
When we lived in the Middle East my then young dds found a kitten (there were lots of semi-feral cats about) in a truly dreadful state. She was incredibly thin and her paws were bleeding as if she'd been trapped somewhere and scrabbling to get out.
TBH I didn't think she'd even last the night, but she did. Next day I took her to the vet, a lovely Egyptian chap who said he'd never seen such an emaciated cat and advised putting her to sleep. But I couldn't, since dds were waiting outside and would have been devastated.
He gave her vitamin shots etc. and she slowly recovered, seemed very contented and enjoyed cuddles.
However we had to,return to the UK a few months later, and it was out of the question to leave her behind. So we spent a fortune on flying her back and 6 months' quarantine. She came out of there sleek and glossy - though her front legs were always stiff, presumably because of early malnutrition.
She settled very well with us again, and since our house is quite a way from a busy road, I had no qualms about letting her out.
But she lasted just 2 months before being hit by a car.
It was awful, esp. for dds, but OTOH she'd used up most of her 9 lives already and would certainly have died if dds hadn't found her and we'd taken her in. So I couldn't feel guilty about it.