You can’t teach an old dog new tricks.
You can’t have your cake and eat it too.
You can’t fit a round peg in a square hole.
You can lead a horse to water but you can’t make it drink.
Your Guess Is as Good as Mine
You can’t unscramble a scrambled egg.
You can catch more flies with honey than with vinegar.
You can’t always get what you want.
Your guess is as good as mine
Your name is mud