what if i forget my old password then what

Then you will lose access to those notes. You can create a new Notes Password, but the notes using the old password will continue to require it.

Click here-> How to reset your Notes password on your iPhone, iPad, and iPod touch - Apple Support

To unlock a note, you need to enter the password that it's locked with. If you forgot the password, Apple can't help you regain access to your locked notes. You can create a new password, but this won’t give you access to your old notes. A new password will let you password protect any notes that you create going forward with the new password.

Assuming that you want to open the note, your ONLY option is to remember and enter the password.

There is no other recovery process or workarond.

There is nothing that anyone else can do to help. Sorry.

The “good news” is that there’s no limit on “failed attempts” … so just keep on trying.

As I posted above:

... . If you forgot the password, Apple can't help you regain access to your locked notes. ...

FYI, your notes are already encrypted and VERY WELL protected by your iPhone’s passcode … and which CAN be restored.

Adding an additional Notes password puts the entire burden of “Password Management” of the note password on the user.

Apple “assists” users by providing the opportunity to add a “Hint” during password entry … but that’s as far as they go.

No. It’s intentionally VERY secure.