"May-December romance" or "May-September romance" is a romantic relationship where one partner is a lot older than the other. Example: The man and his girlfriend were involved in a May-September romance.
“Rub it in” means “to talk about something you know will upset or embarrass someone”. Example: I know I made a silly mistake, but do you have to rub it in by talking about it all the time? Origin: This idiom is a short form of the phrase ‘rub salt in.
“Shoot yourself in the foot” means “to do or say something that causes problems for you”. Example: When Michelle got drunk before her final exam, she shot herself in the foot. She failed the test! Learning English can be fun!