Nothing you do in life will be harder than parenting. You're solely responsible for the health and well-being of a tiny, fragile, irreplaceable human being, and there are no save points or do-overs. It's as serious as it gets.
Of course, there are times of resplendent joy, and it doesn't come without unique love, but it's trying, so very trying. So it's understandable for parents to hit low points along the way, especially single parents.
Luckily, the world can often help lift them up.