The extreme form of nostalgia where you remember a better time to the point that it makes you feel sick. I used to sometimes dream about a former very happy time in my life, and waking up fell like sinking into quicksand and drowning, the realisation that you're in current reality and there's no way back.
Alternative would be deep dread, like you're about to fail your university course or something with the largest stakes up until that point in your life, that's a dark feeling.
The thing is dread, shame, etc all fade as you get older because you've already lost games with high stakes and your life is worth less generally. You've taken hits and they don't hit, the one that never leaves you or gets better is painful nostalgia.
