mental instability vs neurosis

mental instability

neurosis

noun
  • A mental disorder, less severe than psychosis, marked by anxiety or fear which differ from normal measures by their intensity, which disorder results from a failure to compromise or properly adjust during the developmental stages of life, between normal human instinctual impulses and the demands of human society. 

How often have the words mental instability and neurosis occurred in a corpus of books? (source: Google Ngram Viewer )