GA8 pilot reported a taxiway excursion during night operations.

Narrative

After landing on Runway XXL; I was instructed to taxi via [Taxiway] XX; [Taxiway] XY to parking. In the airvan; my windshield and side window had frosted up on the inside after landing due to the extremely cold weather and two of us sitting in the front of the cabin. I took a moment to clear as much as I could on my side with my rag; but it was still quite frosted over. I was having difficulty seeing the taxiway center line with the blowing snow and darkness; but I could see the glow of the blue lights. As I turned from Taxiway XX to [Taxiway] XY; I didn't realize there was one missing taxi light at the corner and I had turned prior to the turn onto Taxiway XY. I immediately realized my mistake in turning prior to the turn in the taxiway and immediately initiated a turn back towards the center of the taxiway. However; the edges of the taxiway were icier than the center of the taxiway; and I was unable to complete a full turn before the aircraft slid off of the edge of the pavement about 4 ft. Due to several inches of snow I was in need of assistance from a tug to return my aircraft back to the taxiway before continuing to taxi back to parking.I believe there were many causes that led to the taxiway event. The darkness; blowing snow and frosted windshield/window made my visibility extremely limited. This airport and taxiways are still rather new to me; and especially during night operations. The iciness of the taxiway edges was the final cause; as I slid the last few ft. even after realizing my error.Completely stopping on the centerline until I had unrestricted visibility through my windscreen may have helped; in addition to the taxiway light being replaced and the taxiways sanded or de-iced in some fashion.