When you create a signing request for your SSL certificate inside of WHM, a private key is also generated and stored on your host.
You can locate the private key by navigating to the following location.
WHM >> SSL/TLS >> SSL Storage Manager
The left-most column is called "keys" and can be used to view the private key for youu SSL certificate.
For additional information, please see our documentation regarding the SSL Storage Manager section of WHM.