You want to use your google authentication, but it says email exists. It means that there is existing account registered with this email address.
1. If you want to login using email but forgot the password, use Forgot Password Button in login. You can follow these steps Reset Password.
2. After successfully changing your password, you can Link your google account to this account by accessing User Settings > 3rd Party > Google Account.
3. Congrats now you can login using google account.