Quite a long story but I'll try condense it as much as I can. I'm a Male [21] and my father died when I was 4. My parents were separated before he died. I took my uncle - my father's brother, as a father figure and called him dad, since he was there for me since then. He is married to a woman since before I was born, and my mother and her didn't really get along. Uncle's wife was very unpleasant to my mother, but because I called my uncle dad, I started calling his wife mum out of respect. Not only that, but whenever I visited my uncle and saw his nuclear family with his wife, I felt quite left out and wanted to be part of it. My mother wasn't happy with me calling my aunt-in law mum, but she tolerated it.
When I started to grow older, she started to tell me reasons why she and my uncle's wife disliked each other (they are civil now though). Uncle's wife was very impolite to my mum in many ways and always used to make my uncle pick between her and his family. I knew about this but still called her mother, but then a while ago, my mother revealed that aunt-in-law had been instrumental in the split between my parents before my dad died. I was gutted and since then I've been re-evaluating calling her mum. Firstly because the more I grow older, the more I really value and love my mother and realise that you only have one mother. I also find it weird to call another woman mother who never really did much for me in life, secondly, she was partly responsible for the split between my parents, which could've meant I would've had siblings and my father possibly be alive (but who knows).
I've told my mother that I want to stop calling my aunt-in-law mother, and she's advising me to do what I wish but is happy that I've come to the realisation that it's weird. Even though aunt-in law was a right cow to my mother and me when I was younger, she really values me calling her mother (she has three kids btw), and it's something that I've done since I was 6/7. I don't want to hurt her feelings and keep calling my uncle dad and then her aunty. I feel like I'm going to be subjected to calling her mother forever even though I don't want to. But I'm very stuck. I've been finding alternative names for mother but can't find any. I've posted this here because its a mum forum and I feel like this would be understood better here.