Try running orphan user script in each and every user databases. This will drop and clear the anamolies in the user-login conflicts. As you are a sysadmin, you can still be in the group and you will have all the privileges on the system. I want understand why you want to be a user again. Recreate all the database objects with "dbo".

-- Orphan user Security Fix:

DECLARE

@username varchar(25)

DECLARE fixusers CURSOR

FOR

SELECT UserName = name FROM sysusers WHERE issqluser = 1 and (sid is not null and sid <> 0x0)

and suser_sname(sid) is null and [name] in (select [name] from master.dbo.syslogins)

ORDER BY name

OPEN fixusers

FETCH NEXT FROM fixusers

INTO @username

WHILE @@FETCH_STATUS = 0

BEGIN

EXEC sp_change_users_login 'update_one', @username, @username

FETCH NEXT FROM fixusers

INTO @username

END

CLOSE fixusers

DEALLOCATE fixusers

GO

Thanks,

.A.