I forgot my Apple ID, how can I cancel my iCloud monthly subscription?

You will have to recover your AppleID to do so.

See -> If you forgot your Apple ID - Apple Support or How to use account recovery when you can’t reset your Apple ID password - Apple Support

If you cancel the card, your account will go into arrears with Apple which could hurt your credit. As far as Apple will be concerned you still owe them money every month until you cancel the subscription. Call Apple support if you cannot complete the automated account recovery.