Forgotten apple id (plz help)

So I just got an iPhone from a relative that I have unofficially used for a while. I had created another apple id (not this one) to use with it, but have by now forgotten the email and password associated with it. I tried to remove it but couldn't, so I tried restoring the device, which also didn't help, so now I'm stuck on the activation lock screen. I have tried every password and email I could think of but none worked. How can I find the apple id email/password and/or remove the activation lock?