Thank you for the details of the issue you are experiencing with Keychain Access on your Mac. I found a couple of articles I suggest for the issue you described.
Your keychain uses your the same password as your login password. If it is not accepting that password, I recommend running the Keychain First Aid using the steps in this article:
If you are still repeatedly seeing a prompt to enter your Keychain Access password, you may need to reset your keychain. This will create a new keychain so any saved passwords on the old keychain will not be available, and you will need to enter them when accessing sites for the first time again. Follow the steps in the article below to create a new keychain:
Thank you for using Apple Support Communities.
Where I work we have to change our login password every 45 days so when the password gets changed the keychain passwords become obsolete and many users forget what that password should be. I've tried the solution mentioned above, but it won't accept the old password. I can enter the new password in all fields and it seems to take it, but the next morning when I log into the computer again, all of the 'keychain password' prompts show up again. Can someone please tell me what else I can try?
Thanking you in advance for any solutions to this issue.