Forgot my old iPhone password, what can I do?

If you don't remember your old iPhone passcode, you will need to erase your iPhone with a computer, using iTunes (Windows) or Finder (Mac), by following the directions in this link --> If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

It will erase your data off the phone.

Yes, ALL. You'll be able to restore your most recent backup to your phone assuming you've been backing up your iPhone. Backups HAVE to happen before something goes wrong.