Imagine if you knew your mum regretted having you, you'd be heartbroken
As pp have pointed out, many don't have to imagine that.
I don't recall ever being heartbroken about being unwanted as a kid. This is maybe because large parts of my community often spoken of kids like many talk about jobs - it's part of life, some of it's crap, some of it's great, and I just kinda accepted that my mother hated being a parent just like some people hate their jobs. As a kid, my main aim was moving away so we'd both be free.
I do remember being upset at adults who tried to push on me that all mothers deeply love their kids, that all mothers think their kids are amazing and beautiful, that I had to understand that she's going through a hard time, but really loves me. Those were the only things that made me feel that my mother obviously not must be my fault. That if my mother thinks I need to be bleached then I must be really really ugly because all others think their kids are beautiful. As an adult, most of frustration is at communities and institutions that failed us both and adults who put these platitudes and images of what they thought motherhood should be before reality. I'm glad I had some reality then.
I do think there are issues with ruminating on the negatives too much, it's not good emotional care, and there are obviously those who say horrible things to their kids, but I think there is a need for a space to talk about these feelings and that there is better counter to those that feel low than going on about feeling amazing. I think a lot of media goes to these extremes (much like with work more and more I've noticed, it seems to be either your passion or misery, can't just be a job).
Personally, while there are lows and highs, most of my parenting life is fairly neutral. My neutral has gone up over the years through getting better at parenting and at caring for myself, but I wouldn't call it amazing most of the time and loving them has never solved any problems as my problems have nothing to do with loving them. I'm not sure loving kids should be seen as a solution to anything.