I believe the word itself is a verb meant to convey the idea of something shrinking or decreasing. By tagging four simple letters to the end (able) we have an adjective; another four (ment) will give us a noun. An "ly", as usual, transforms the word to an adverb. A few short words ending with "return" will even give us a law. I, however, provide you with the embodiment; the state of being.
I may have had many parts of speech attached to my life before: complex, exuberant, intelligent, competent - perhaps even ubiquitous. Yet another word now runs through my mind unimpeded. I am diminished. My life is diminishing. I am diminishment itself. My star has dimmed. I have no need of diminutives. I have become dimwitted.