Assuming you can log in initially (on start up) perhaps it would be better to try to change your password the old fashioned way. After initial login go to the Apple Menu top left, open System Preferences, go to Users & Groups, select yourself in the side bar and you should see change Password on the right, click that. Now if you are running the latest Operating System you will see a box asking if you would like to use your iCloud Password to log in. I would not choose this for security reasons. Click Change Password. Now you will need to enter your current password, if it's accepted all is good. Unfortunately you now need to think of a new password again. Personally, just as an idea I favour sentences. For example; Dad'sHairIsGrey. Or IDontLike3Fish. These are easy to remember, each new word starts with a capital letter but just as easily you could make each word end in a capital, they are hard to break but simple to write, adding a number makes them even harder to crack but don't use symbols.
Let us know if the above works for you.
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