To have second thoughts is to start doubting a decision you made. She began to have second thoughts.
The doubts come after the decision, which is what separates it from hesitating. It is nearly always used of something not yet irreversible.
1phrasehave second thoughtsTo start doubting a decision you made.
“He signed the lease and began to have second thoughts on the way home.”
have second thoughts is a phrase.
Practised in Idioms