How to fix Keychain on macOS Sierra
The keychain may still be using the old password. In order for the act of logging in to unlock your keychain the two (user account and login.keychain) need the same password. I would recommend turning off in Keychain Access the settings that will prompt you to enter your long password. With your login keychain selected on the left, under the Edit menu select "Change Settings for keychain login" and uncheck both boxes.