Now you need to verify if you really are the original owner of the account. Signing a message from an old address would work.
I have some old addresses from 2015 or even 2013 to sign. But that wouldn't proof anything. I'm not the only one with some old addresses.
That doesn't make sense.
Your private key belongs (and should) to you and you only. (you need to have had posted that address sometime in the past in the forum, so we'll know if that's really you)
Might be an address you used to join a giveaway, sig camp. whatever.