Your username is the registered email you used to make your purchase (normally your Paypal email). If you are having problems logging in, the absolute fastest way to fix this is to RESET YOUR PASSWORD using your registered email.
This works 99.9% of the time to resolve your issue.
If you try resetting your password (with the correct email) and still cannot login, then you have permission to open a ticket and support will help you. But, please try to reset your password before contacting support because I’d bet dollars to donuts this will work for you (and you won’t have to wait).