exactly, but is there a way to change it how it was before that made it fill up the whole screen when it wasn't native?
No, there isn't.
Obviously, Windows 10 has made it so that instead of stretching the image, it puts in the black bars to retain the aspect ratio.
There are only two ways to fix this, neither of which actually "stretch" the image like you want:
1. Stop bitching about your FPS and set your resolution to 1366x768.
2. Play in windowed mode.
If you had a monitor with a more standard resolution (ex: 1920x1080, 1280x960, 1280x720), you could just set your resolution to something lower that has the same aspect ratio, therefore maintaining FPS and eliminating the black bars at the same time. But you don't. So...pick what you want to do.
