About cancellation...

You were on an annual fee, paid on the month. For not respecting your contact and cancelling early, you are required to pay an early termination fee. That is nothing unusual. If you wanted a monthly plan, you should have taken the pricier monthly plan, that is also available. That plan can be cancelled for each end of the month.

By taking your plan, you agreed to the terms of that plan, and the terms were sent to you by e-mail the moment you took the plan.

Sorry, but you should read for what you sign up in the future.

As a side note: you posted in the stock forum. Stock is not a software, but a service, but fortunately, the conditions are the same.

You may try to contact customer service to negotiate your way out of your contract. Adobe customer care can be contacted by beginning a secure chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled! If the chat window fails to open, or is non-responsive, use a different device to start the interaction.
Support phone numbers may be found here: https://helpx.adobe.com/contact/phone.html. Please note that phone charges may apply.

For support via Twitter:
https://twitter.com/AdobeCare

(see also here: https://community.adobe.com/t5/creative-cloud-services/how-to-contact-adobe-support/td-p/11875703 or here
https://community.adobe.com/t5/account-payment-plan/how-to-contact-adobe-support/td-p/11843852)

You signed up for an Annual Plan with monthly payments. Annual Plans offer the best price, but it means you're committing to pay for a year. The terms were made clear at the time you agreed to the plan. 
https://stock.adobe.com/plans#plans-faq