If you have your old username and password, you can use that to log in and then click "Account" in the top of your page to access your profile. If you're on a mobile device, click the person icon at the top of the page to see the menu options.
Your username is at the bottom of your Account profile page. You can also use the link from Account to reset your Peer-to-Peer password and to disconnect your page from your Facebook Account.