Hello, last weekend dnd beyond randomly logged me out and when I attempted to log back in I was unable to access my account PoateUwU, and instead it created a new account.
Since I've had the account for a while, I don't remember the specifics of how I created it and can't find a way to reset my password using just my username!
Please assist me in this matter :(

Logins are based on the 3rd party accounts such as Wizards, Google, Discord, Twitch and Apple. D&D Beyond doesn't have its own login.

It seems you used a different login account. Log out and use the original. If you have forgotten the password you may need to go the account's site (Google.com for Google, etc) to login/reset password there.

Is there a way to tell what login account it is? I have tried logging in in MANY ways and nothing is working

No. Only you would know because you made the account.

But, maybe you can contact support. However, they cannot just tell you what email address was used (Data Protection) but they could tell you if it was a Google or Twitch or whatever. You can contact them using the Contact Us link at the very bottom of the page.