look back vs unforget

look back

verb
  • To think of (past events) in retrospect. 

  • Used other than figuratively or idiomatically: see look, back. 

  • To reconsider or regret. 

unforget

verb
  • To not forget; also, to remember again after forgetting. 

How often have the words look back and unforget occurred in a corpus of books? (source: Google Ngram Viewer )