Changing current anuual plan to monthly

that's going to be difficult.  if you'd posted 2 months ago when you realized your error, you would have been advised that even after the 2 week cancellation period adobe support is generally very understanding of that error and likely to change your plan with no penalty.

at this point, it's likely they'll be understanding (for a few reasons), but you can try.  there are 3 ways to contact adobe; chat, phone and twitter delineated by @Abambo above.

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true

I do not see a lack of clarity, as the conditions are shown on the web site and you explicitly accept the terms of the contract and the cancelation terms. And what is lacking clarity when you subscribe to an "annual, paid monthly"?

Contact Adobe Customer care, and see what can be done for you:

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)