I mean, you take your average Russkie, we all know how much guts he's got. Hell, lookit look at all them them Nazis killed off and they still wouldn't quit.
It was really over in December 1941. Usually what you hear is the Russians stopped the German advance within sight of Moscow. That statement is a bit misleading. The December counter attack did more than stop them, it devastated the German Army. They suffered over a million casualties and were reduced to this limited operation the following spring focusing just on the southern edge of the front. Stalingrad was just when it became obvious to everyone else that Germany could not win.