Memory is not always a reliable tool. Sometimes you are absolutely sure you remember something correctly, even when you don't. I have wasted a phone by entering the wrong PIN-code three times; at the time I was absolutely sure it was the right code, but a day later I realised what my code really was. I recommend meditating before going to sleep; clear your mind of what you currently think is the correct passphrase and ask your mind what the real passphrase is. Even if you don't "get an answer" during your meditation, since you have been focusing your mind on it, the next morning you may suddenly get it. Also stop re-entering the same passphrase that isn't working, you are only confirming an incorrect memory and making it stick.