Losing life
expiring, sinking, moribund, on one's deathbed, passing away, going, perishing, failing, fading, near death, at death's door, in extremis (Latin), terminal, withering away, paying the debt of nature, not long for this world, fated, done for*, booked*, giving up the ghost*, cashing in*, on one's last legs*, with one foot in the grave; see also sick.
Antonyms immortal*, living, reviving.
Becoming worse or less
declining, sinking, receding, decreasing, disappearing, diminishing, dwindling, dissolving, disintegrating, vanishing, failing, fading, ebbing, decaying, smoldering, recessive, overripe, decadent, passé, doomed, neglected, superannuated, done for*, fizzling out*, in its death throes.
Antonyms growing*, eternal, enduring.