Reobbot your computer and hold down the Command and R keys whilst booting. This will take you to your Recovery partition. Once there, select Terminal from the Utilities menu and type:
...a small app will open alllowing you to change the password for a selected user. After you've entered your new password (twice) and given yourself a good hint, click OK (or Done or whatever) and reboot your computer.
You'll now be able to use your new admin password with your admin account.