Global WooCommerce

Hi
I am new WooCommerce and WordPress developer.
I am currently developing a custom payment method, after checkout process (place order button) I redirect the user to my website to fill the credit card form when the process is done I return the user to thank you page or failure page in the WooCommerce website (online store). The problem is when I want to empty the cart I couldn’t use the global $woocommerce due to an undefined global variable, even when I try to put it in the PHP session and read it again, I got this error __PHP_Incomplete_Class Object.

any one can help or have an idea to make this run.