I so know this feeling! I still remember exactly how I first felt it; I was six, pondering the universe and mentally zooming into a tear in a wallpaper, down to the subatomic particles level, then I zoomed out, but for the first time didn't stop until my mind has encompassed the entire spacetime, the entire, timeless universe, with me but a infinitesimal speck in it. Interestingly though, I quite liked that feeling, in a bit perverse way perhaps; I learned to invoke it almost on demand and did it quite often, especially when I was upset with the world around me. It brought serenity uncomparable to anything else I've experienced, and, at times, welcome detachment. I have recognised it as sad, but it was serene sadness.

Years later I've found out that I'm clinically depressed and perhaps that's why I don't feel the sadness so deeply - it's not much lower than my mood set point. BTW, comparing this feeling to the sadness of a close being passing away is like apples and oranges - they both have a completely different flavor to me.

I still invoke it from time to time; for the serenity, sometimes for the detachment, and oddly, sometimes get sad to get angry and gain some motivation to change the world. I've found that attaining the state is now harder than it was when I was a child.

BTW, interesting tip about the poetry; I've been wondering why it's not as alluring as it used to be, and perhaps I don't spacetime out that often anymore.