How can i get my old notes password and open the notes which is already locked in my phone and that is already forgotten password

You can't. If you didn't write the locked notes password down or create a locked notes hint when you locked the notes, you will never be able to unlock them until you enter the correct password.

Things like this, which are important to keep track of, should be noted down somewhere for future recall.

Something as simple as this can be worth it's weight in gold:

Did you not read any of the posts above your post? The answer isn't going to be any different for you.

1. If you can't remember, nor di you write the locked notes password down, nor create a locked notes password hint, then the notes will remain locked forever.
2. Apple can't help you.
3. We can't help you.
4. There is no way to retrieve or reset the password for notes which are locked by a password. None.

i forget my notes password