When you have forgotten your password -- whether it be the account's main password or the (optional) secondary password that controls whether you can send Bitcoin from your account -- you only have one option.  You need to recover the password.  Blockchain.info can't help you because they don't know the password.

There are several different password recovery strategies, but they all boil down to identifying likely passwords and then testing those passwords, one after the other until you find the right one.  We recently published a "how to" article that walks you through how to choose the passwords that are most likely to work, and then how to test them.  The article is here: https://cryptoassetrecovery.com/blog/recover-lost-blockchain-password/

The only wrinkle in your case is that you need to test the secondary password rather than the main password.  (You would probably figure this out on your own, but) all you need to do is login with the main password and then attempt to send your Bitcoin to a new account.  Blockchain will then prompt you for your secondary password, and you can begin to test the most likely options.

Good luck!