can't change my administrator's password

Have you tried the steps in this Apple note to change your administrator password?

Here's a quick tutorial on resetting a password outside of Mountain Lion. If you don't have a recovery console you'll use the Internet Recovery option (See http://support.apple.com/kb/HT4718 – for information regarding booting to this partition)

1.) To boot in Recovery system, press Command and R keys in boot and hold the keys until you see the Apple icon.

2.) When it starts, select Utilities > Terminal, and type:resetpassword

3.) Select the Username and type in the new password.