On the WP Discourse SSO Provider settings page there is a ‘Path to your Login Page’ setting. It sets the login path that is used for SSO. Try entering the path to the login page that you want users to be redirected to when they login through discourse.mysite.com. The setting takes a path, not a full URL. It should look something like this: /my-account.
This should allow users who visit /mysite.com/wp-admin to be redirected to /wp-login.php, and users who initiate login visiting your forum to be redirected to your custom login page.