I don't know if it'll help, but decadent, although it's typically used as a positive, ("decadent chocolate cake", "decadent jewels", etc.) was originally a term that meant "decaying" or "declining"

 https://www.merriam-webster.com/words-at-play/decadent-word-history-luxury-or-decay

The feeling you're describing isn't exactly a feeling of decline or decay, but there is a bit of "not all there that could be" that is at least tangentially related to decay and decline.

I don't know if it'll help, but decadent, although it's typically used as a positive, ("decadent chocolate cake", "decadent jewels", etc.) was originally a term that meant "decaying" or "declining" (M-W)

The feeling you're describing isn't exactly a feeling of decline or decay, but there is a bit of "not all there that could be" that is at least tangentially related to decay and decline.