Overtightened a flexible hose

I swapped out the filler valve on the toilet because the old one was hissing. The new one has a brass inlet. I had hand-tightened and given it a quarter turn on the first go, but it leaked straight away (quite a bit). So a bit more turning, bit more turning.... Then I turned the washer over cos it was 3 years old and a bit crushed. Its leaking a bit and basically I think I've overtighten it. its got to be way more than hand tight plus a bit, although that's subjective so who knows.

Anything I can do to rescue it ? I suppose it might need a new 1/2inch to 15mm hose but a simple washer would be better.

cheers
BB
/didn't plan on doing DIY today, grrrrr

Replace the washer.
I always replace them when re-using.