Other forms: second thoughts
Reconsidering a decision you've already made is called having second thoughts. You may have second thoughts about seeing a scary movie if after five minutes you're already covering your eyes.
When you rethink a choice, you have second thoughts. You might request vanilla ice cream but have second thoughts when you see your friend's hot fudge sundae. Or you might sign up for a Spanish class, only to have second thoughts because all your friends are taking Arabic. You can also use the singular version of this term, especially in a sentence like this: "On second thought, I think I'll have the hot fudge sundae."