If your Form was made through the Applet and not from a Group/Team site, I'm not sure if there is a way to get to the source sheet directly, you'd have to rely on downloading it.
One thing you could try to get to the data more easily would be creating a List replicating the structure of your Form, copying over what submissions you already have, and creating a Flow to direct responses to your List. This makes working with Form data a little more reliable in my mind, and would accomplish your need if the source sheet proves elusive.
