To retrieve a forgotten Apple ID username, go to the My Apple ID page on the official website, select Find Your Apple ID, and type your name and present or previous email addresses. Select the Apple ID that matches your current email address if the result shows multiple Apple IDs.
After finding your Apple ID, follow the instructions to gain access to your account. Change your password, and update your settings afterward if you cannot recall your password. However, if you know your password, select Cancel, and proceed to log in to your account.
If the steps above do not work, use your Apple mobile device to find out the Apple ID signed in to Apple services, such as iTunes and iCloud. Press Settings, choose iTunes and App Store, and check your Apple ID at the top of the screen. Alternatively, select iCloud in your phone's settings to see your Apple ID.
Another method to check your Apple ID is to press Settings, Messages, and Send and Receive. The FaceTime and Game Center sections in your mobile device's settings also show your Apple ID on the uppermost part of the screen.
Type your Apple ID and password to access your account. Finally, update your Apple ID email address after signing in to ensure you don't forget it.