what if I forget my password and cellphone number?

It means you need to write things down in the future. Your account is toast.

If you did not set up a recovery email, then the account will be lost.

Unless you have set up a recovery method you won't be able to get it back. This is how to recover your account if you have password/sign in problems: https://help.yahoo.com/kb/SLN2824.html

It means you have dementia. Seek help.