Idiom: Turn over a new leaf
OED definition: to make a fresh start, to become a better person
Example: Since becoming a father, he has really turned over a new leaf
This idiom comes from the fact that in the 16th century, pages in books were known as ‘leaves’. So, when you ‘turn over a new leaf’, it is like turning to a new, clean page in a notebook. Traditionally, this would be in order to start a new lesson, however, with time, this has come to mean a new stage of life or the opportunity to start anew.