I have a friend w/ this aspect in her 12 house in Sag. She's a teacher and author. She's worked really hard, but it seems like she's never got enough "success" to her satisfaction. I think maybe she feels unseen in some ways?

I think I can shed some light on how you "know". You have a Scorpio Sun located in the 12th house. Both are water-based and water signs are linked to psychic sensitivity. It's probably the Scorpio element that allows you to "know" stuff as that sign possesses strong intuition.