Do you get email receipts for your subscription purchases? Search for one of them in your email account. In it, the Spotify username you're paying for is shown:
With that email address, you should ask for a password reset. That should give you access to your account again.
After that, be sure to use that username to login to your account. It's the one with the Premium features.
Hey there you,
Yeah, you! 😁
Welcome - we're glad you joined the Spotify Community!
While you here, let's have a fun game…