I left home when I was 20 in order to go to Uni and I was obviously still in contact with my mom and saw her 3-4 times a month, but she was enough of a distance away from me that she wasn't overly involved in my life. Prior to that, when I was growing up I had always just told myself I just had a mum who was "really strict" and didn't really give it much thought. (Now I know I just didn't want to face what kind of mother I had).
As the decades passed I still lived near my mum and our lives ticked along nicely. To everyone else we looked like we had a pretty normal mother and daughter relationship.
Then when I was 40 years old she moved house and moved a lot closer to me, and her suddenly being a much more active part of my life, and seeing her at least 3 times a week, made everything start coming back to me again. This time I saw it all differently though as I was a mother myself by that point, and I struggled to cope with the idea that a loving mother could treat her child in the way she'd treated me. My entire viewpoint of my childhood had shifted.
When she started being actively in my life again, I began seeing her nasty, manipulative and controlling behaviours all over again and I realised it wasn't very easy to keep all the bad memories of my childhood pushed to the back of my mind when my mother was in my life every day. I felt so much anger towards her.
Ultimately I just couldn't suppress it anymore and I had to face it.
Going NC isn't easy, but everyone has a breaking point where they realise that emotionally they just can't cope with having someone in their life anymore.