If you build so many websites that you eventually forget not only your password but even username or maybe even email you used to install WordPress, here is how you can find it (provided you still remember how to access to cPanel where your site is hosted):
1. Go to yourwebsite.com/cpanel and login using your hosting login details.
2. Under “Databases” find “PHPMyAdmin”
3. Find the name of your WordPress database installation in the left column and click on it.
4. Click on wp_users either in the left column or in the center of the page.
Next you will see the list of all users, their usernames and their emails, including your own. You now can sign in or reset password using your email address if necessary.