I've paid for your app but don't remember the username and can't log in
If the system does not recognize your email it means that you either have used another email at registration or have not createded an Ultimate Guitar account at all.
Simply click on the "Sign up" button in the account's section of the app.
Creating an account is free.
Note: all apps' subscriptions are linked to Apple ID and not to Ultimate Guitar account. So don't get all riled up; the subscription you purchased is in good standing.