My mum had an accident and long story short, after a few years of semi-recovery and then decline (with my dad doing a brilliant job of looking after her), developed a form of dementia (fronto-temporal) as a direct result of the accident and ended up having to go into a nursing home (and died 1.5 years later).
I was fully supportive of dad's decision.
Subsequent to that, my dad met an old Uni friend of his (so c50+ years ago) at a reunion (Uni was in the country that mum and dad were from). She'd been his best friend's girlfriend at uni
- although she went on to marry someone else. They got together subsequent to that reunion: she was widowed and although my mum was still "alive", she had in practice "gone" 
I was happy about it as his lady friend knew about Mum & Dad's history - "the hottest thing on campus"
and that they'd remained truly in love, a golden couple as one of my friends described them. She in no way tried to replace Mum's place in Dad's heart.
I'm just glad Dad has someone to share his life with. She lives in another country so they only meet up for a couple of months at a time (and at the moment can't because of Corona virus restrictions). I might have felt differently if she'd been a gold digging harlot younger than me rather than a kind lady my dad's age 
Mum was and is special and Dad and I still share happy memories of her (with time - she died 8 years ago - it becomes easier to remember the "real" person and the good times
, rather than the sadness of the last few years). His new" lady friend has brought him friendship and companionship and doesn't try to compete with mum's memory.
I'm not saying you're right or wrong to feel the way you do: just giving a different perspective from my experience 