have second thoughts
to start having doubts about whether a decision you have just made is the right one

[TahlilGaran] Dictionary of Contemporary English ▲

doubt to think that something may not be true or that it is unlikely:
‘Do you think she really is eighteen?’ ‘I doubt it.’
There was so much noise that I doubt if anyone slept.
be doubtful/dubious /ˈdjuːbiəs $ ˈduː-/ to doubt that something will happen, is true, or is a good idea:
Economists are doubtful that the situation will improve this year.
‘You can eat the whole fish including the head.’ Janey looked dubious.
When my husband suggested emigrating, I was dubious at first.
Mum looked doubtful when I suggested having a party for all my friends.
have reservations to feel that a plan or idea may not be good because you think there may be some problems with it:
Health care professionals had reservations about giving both vaccines together.
have misgivings to feel worried about doing something, because it may have a bad result:
Ralph had serious misgivings about changing his career at the age of 50.
have mixed feelings to be unsure exactly how you feel about something or someone because there are both good and bad things about them:
I have very mixed feelings about moving house – it’s exciting but I’ll miss this area.
have second thoughts to start having doubts about whether a decision you have just made is the right one:
I liked the dress in the shop, but when I got it home I had second thoughts.

[TahlilGaran] English Thesaurus ▲

have second thoughts
if you have second thoughts about something, you change your opinion or start to have doubts about it.
You're not having second thoughts about coming to Brighton with me, are you?
(often + about)

[TahlilGaran] English Idioms Dictionary ▲