have second thoughts about (someone or something)

To reconsider someone or something; to begin to doubt a previous determination about or commitment to someone or something. Why are you hesitating? You're not having second thoughts about him, are you? A: "Where's the bride?" B: "I don't know. Maybe she had second thoughts about getting married." Five minutes ago, you thought this was a great idea! Why are you having second thoughts about it now?