Recover an old account's name

Hello,

I have a problem. I have an old account created I guess in 2015, and now I play from another account created in 2018. The thing is in the old account I have a character with a specific name. I can't log in because I don't remember the old account name, but I know the email and password... I need to delete it, or recover it, because otherwise I can't use that specific name that is inside that account which I don't remember its name. Sorry for my bad english... Please, if you can help me, how can I find out the name of that old account? Thank you!