"Buyer's remorse" is a phrase used to describe the feeling of regret or dissatisfaction that someone experiences after making a purchase. Full definition
I put off car shopping because I hate the pressure to buy that you normally get at many dealerships; the feeling that if you aren't a good negotiator you will be overpaying; and having the feeling of buyers remorse once I sign the papers. (cars.com)
Political spin aside, if this first week is a clue about what «change» looks like, Albertans might be feeling buyers remorse after April 23. (daveberta.ca)
You'll only suffer from buyers remorse till you start using it. (goodereader.com)