Not (very) applicable if you have a strong gender identity: I'd always been fairly confused as to whether I was a woman or a man, neither one really fit quite right. Five years ago I met my first person who identified as genderqueer, but generalizing from that (one) example I thought it a mere label - I didn't see how their behavior diverged substantially from their sex given gender. Last year or so, The Third Alternative, Keeping Your Identity Small, and applying being a linguistic descriptivist to my broader life came together. I'm a person, more or less human, and that's it. One can choose null (but of course, the vast majority of acquaintances won't notice...).
Not (very) applicable if you have a strong gender identity
I think this one is also applicable if you do. Anyone who feels the need to do certain things, or avoid other things, against their actual wishes in the matter, because their "gender identity" tells them, "a Man must ---" or "a Woman must ---" would help themselves by learning this lesson.
Sometimes our minds suddenly "click" and we see a topic in a new light. Or sometimes we think we understand an idea, think it's stupid and ignore attempts to explain it ("yeah, I already know that"), until we suddenly realize that our understanding was wrong.
This kind of insight is supposedly hard to transmit, but it might be worth a try!
So, what kind of important and valuable insights do you wish you had earlier? Could you try to explain briefly what led to the insight, in a way that might help others get it?