Go to help.steampowered.com

Choose “I can’t sign in.”

Select “I no longer have access to my email address.”

When prompted, provide any proof of ownership you still have old email addresses, purchase receipts, CD keys, wallet top-up codes, or payment info (last 4 digits of card, PayPal, etc.).

In the message box, explain clearly that you’re recovering the wrong account and include the correct username / profile link of the one you actually want.

Steam Support will review manually and transfer recovery to the right account if you provide solid proof.