i forgot my passcode immediately and i dont have a backup but i know my old passcode

The only way to unlock your iPhone is, follow the steps described in the Apple article:

(< iOS 15.2) -> If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

• If you don't have a computer and you can't borrow one, you need to go to an Apple Retail Store or Apple Authorized Service Provider for help.

(iOS 15.2 and newer) you don’t need to have a computer -> If you see an iPhone or iPad unavailable message or Security Lockout screen

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Temporarily use your old passcode when you forget your new passcode on iPhone or iPad

Temporarily use your old passcode when you forget your new passcode on iPhone or iPad - Apple Support

• In iOS 17, iPadOS 17, and later, Passcode Reset helps you maintain access to your phone if you forget your new passcode.

Or see: