Use gmail's account recovery process.
I am confused though. You say you were able to change the password. Did you do that using Gmail's password recovery? If so then that is what you need to sign on. Make sure that you are typing the password exactly as you entered it and make sure your caps lock is not on (unless you had it on when you set the password).
One thing you can do to help with such things you can consider - it isn't the safest thing in the world to do but you can type the password you want to use into notepad, copy it to the clipboard, and then paste it into the password reset box using <shift><insert> so you know for certain what you set it to be. You can also use the same techinique to login, but I don't advise doing this on a routine basis.
If you do have problems with passwords on a routine basis I strongly urge you to consider a password manager such as LastPass or KeePassa. Both services offer a free version which will work just fine for almost anyone. Using such a tool you can make sure all of your passwords and long (at least 8 characters) and a mix of upper case, lower case, numbers and symbols without having to worry about forgetting them. All you have to remember is the one master password for the password manager. As you only have to remember one password then, even when dealing with sites which force you to change on a regular basis, you can make this master password complex without worrying about forgetting it. Don't write it down anywhere, however. At most, write down some hints that would tell you the password but would be indecipherable to anyone else.