If you forgot your ESTA application number, you can retrieve your existing application through the official ESTA website.
In general, you can look up the application by entering your passport number and date of birth, or by using passport details such as your country of citizenship, passport issue date, and passport expiration date.
After retrieving the record, it is a good idea to check the current status of your application again.
For a smoother trip, please review your application details in advance and apply for ESTA before your departure.