try to mirror and then knit, if that doesn't work. cut in half and mirror all the surfaces.
I'm not sure why the fill didn't work and I don't have time to open your part to check. You might try breaking up the surface into some smaller, more easily handled pieces, using 3D Sketches to put it some Splines that match up with the surrounding surfaces.
You might also try making the center sections between the two fills longer, so that the fills don't have to wrap around so far. Remember that the ideal surface patch is a square; the more you depart from that shape, the harder you make it for the software to work. Stretching out the two center sections isn't hard. Wrapping back into the corners of the two fills is.