I tried to set it in Keychain Access but the item in the Edit menu was greyed out. In High Sierra, my login keychain was stuck on the previous password.

Click OK when done, then quit Keychain Access.

Enter the same password in the Verify field. This is the password you're now using to log in to your Mac.

Enter the new password of your user account in the New Password field.

This is the password you were using before the password was reset. Enter the old password of your user account in the Current Password field.From the Edit menu, choose “Change Password for Keychain 'login.'”.Open the Keychain Access app, which is in the the Utilities folder of your Applications folder.If you do know your old password, use that password to update your existing login keychain: If you don't know your old password, the solution is to create a new login keychain. That's because your login keychain is still using your old password. It might also tell you that the system was unable to unlock your login keychain.

You Mac might then ask you to update your keychain password or enter the password of your login keychain. This can happen after you or your Mac administrator resets the password of your macOS user account. To resolve this, you can either replace your keychain and create a new login keychain, or you can update the old keychain with your new password. The password of your macOS user account might not match the password of your login keychain. Synchronise your user account and keychain passwords!

Mac login keychain password not working