Learn English – Antonym of “Megalomania”

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Is there a single word that means roughly the opposite of "Megalomania"?

I tried separating the word in to parts, and finding opposites of the individual parts, and the word I came up with was Micromania. But looking that up led to a definition very different from the opposite of megalomania, so now I'm stuck again.

Working off of this definition of megalomania: "delusion about one's own power or importance", What word means "delusion about one's own (perceived or actual) lack of power or importance", especially when it's to the severity of being a psychological disorder.

EDIT: After reviewing the answers so far, and playing around with adding prefixes and suffixes to existing words, something else I came up with that somewhat describes what I'm looking for is Autonihilism. Nihilism being the belief that everything in life or in the world is meaningless or worthless, while the "auto-" prefix narrows the focus from the whole world or all of life down to just ones-self. But I was hoping to find a word already in use, rather than one I created myself (and I'm not entirely certain that "Autonihilism" actually means what I described it to mean, except as far as I get to choose what my own created words might mean. But the description might still help others understand more clearly what I'm looking for)

EDIT 2: Comments regarding "abjectness" led me on a search through Thesaurus entries until I found "Apathy". A person with the condition described by the word I'm searching for might not consider their situation to be abject, merely a fact, just like a megalomaniac might not recognize the abjectness of their megalomania. The apathetic indifference of the affected individual to their own condition, simply treating it as a fact of life, is relevant. An outside observer might consider their condition to be "abject" in severity, while the individual themselves would not. In this sense, it's very different from something like abjectness or depression, where the affected individual might very well recognize the harm that comes to them from the condition. The word "Apathy" itself seems to relate to what is going on around the individual, while the word I'm looking for relates to the individual's view of the individual themselves, and so doesn't quite fit, when used alone.

Best Answer

You can say that person has an inferiority complex.

From the Wikipedia article:

An inferiority complex consists of feelings of not measuring up to standards, a doubt and uncertainty about oneself, and a lack of self-esteem. It is often subconscious and is thought to drive afflicted individuals to overcompensate, resulting either in spectacular achievement or extremely asocial behavior.