I need some advice with the Humblebundle version of the game and Steam version.

I bought the game on HumbleBundle, and I want to move to the Steam version. I don't know why I bought the game on Humble, but that's beside the point. I can use the product key, you may be asking. That's where the problem was.
I didn't always have a steam account, so I made some throwaway account with no credit card linked to it. I know, bad idea, but at the time I just didn't want to make an actual steam account, don't ask me why. Obviously since I used the product key, I can't redeem it on my new official Steam account.
Unfortunately, I don't remember the throwaway email ID, so I can't login with that.
Is there some way to regain that product key, or at the very least find out that Email ID?
Thanks,

 

I think that once you don't remember what email was assigned to the account, then you're pretty much done & will have to re-buy the game