A lot of people do it for the best. I had a friend whose ex wife made visits difficult. The child, (whom I knew through my niece), grew to hate the Sunday afternoon "outings" and the fuss to get ready and the quizzing when she got back and having to miss classmates' parties and shopping trips and swimming and, and, and - because it's "your day to see your dad"
It was hard to develop a relationship with her Dad whom she did not share Christmas or holidays with, who was not allowed to do playdates who became like those formal uncles who ask you "So how are you getting on at school then?". Awkward
In the end as the child grew older there was always a reason why the visit had to be cancelled. He decided it was best for her if he withdrew and left her to have a stable relationship with her stepdad, so apart from a fortnightly letter, (pre-email), contact was ceased.
The happy ending is that when the DD was older she got back in touch. She learnt the full story, got to know her dad as a person and she is now very much part of the family. She has her own DD now - and they are all very close.
Sometimes leaving your kids is best for them. They know that you are there but any contact is always going to be fraught especially if you have a bitter break-up, if the child was very young when it happened, if you have another parent using the relationship to score points.
As a person you have to move on. What choice do you have? You don't ever forget but ....