As a user has already said delete what you already have and just set it up with your email address:
Open Mail
go to Preferences and accounts
Delete any yahoo account you currently have added.
Click add an account:
Enter your Email Address:
yourname@yahoo.com
Password: Enter your password
Click Sign in.
DO NOT go into the account and go into 'Change Login Settings'. There is nothing you should be doing here, but it sounds from your message that you are changing settings here.
If you follow these simple directions you shouldn't have any problems.