From my observations, in the average person selfishness is more abundant (than "necessary"?) which is oppressed under moral codes. But selfishness has its own life as part of the personality, and always struggles to manifest itself.
“人不为己，天诛地灭”is an answer, an inspiration for this struggle and a permission for its realization. The personality is seeing its own true face after longtime turning away from itself. This is why you shed tears, I think. You didn't seem enlightened at the end.
把注意力放在了自身，就没有了那份据子女为己有的私心
No need. You don't need to feel ownership of your children to begin with. No need to go through selfishness in order to reach unselfishness.
“人不为己，天诛地灭” is therefore for selfish people who think they are not. In yfz9465 words, 虚偽, but in a good way. Truly unselfish people don't need this teaching. And selfish people who are honest about it could get worse with this teaching.
