How can I recover my forgotten iPhone 15 passcode?

Sorry, but no one can help you retain the data if it's not backed up. The Passcode is extremely important to know. If you don't know it, you have no other choice but to erase the phone using a computer. If you had a backup, you could have restored your data from the backup. If you don't have a backup, that isn't an option. The best you can hope for is you were syncing some things like photos, messages, etc. to iCloud and after you erase the phone and sign back in with your Apple Account and Password, you can turn the iCloud switches back on and any data you did sync with iCloud would be redownloaded. Apple cannot help you restore your data, whether you have proof of purchase or not. All the data on your phone is fully encrypted and Apple has no access to the data.

Backups MUST happen before something goes wrong.

Follow the directions in this link --> If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

You must erase the phone in order to reset the passcode.

If you do not have a backup, you are out of luck. You cannot recover what you do not have. Neither can Apple.

Do a backup regularly. You have learned the hard way. Sorry.

You have really had this device since October 2023 and never backed it up? Did you never feel the data on your device was important? What if your device had been stolen, you would have lost all of your data that way too. You have been taught a very hard lesson. The worst part is that you could have created a backup on your device daily by just plugging it in to charge overnight by activating a setting in Settings and your iCloud.