Ok I'll give you the US assistance was utterly invaluable but the Russians never would have gotten fed up. They suffered the brunt of German battlefield Atrocity, particularly as the Wehrmacht retreated from Russia. The Nazis would hang trees full of bodies of dead children hoping to demoralize and raped, murder, and plundered everything Russian they came across. Oh while they nazi's took soviet soldiers as POW, they shot officers outright and instantly, then they would death march and starve the soviet rank and file...so in the end net effect the Soviets got almost none of their POW's back alive.
And even during the Barbarossa invasions...the Russians were insane. Farmers, old men, teenagers, civilians alike volunteered to take up anything from rifles to pointy sticks to fight the nazis. Stalin ordered evacuations and retreats, the farmers themselves burned their own towns and scorched their own fields then took anything they had and attacked the panzers. In Stalingrad, rather than evacuate plant workers would hold the nazi's off at one end of the factory while they put out tanks to join the fight at the other.
You have to remember these were little better than what what the US would call a state militia. And they slowed the Wehrmacht, easily the most powerful army in the world at the time, from just steamrolling right into Moscow for most of a year, the same Wehrmacht that took France in six weeks with almost no losses.
US history books of course mis-remember these events as Stalin ordering millions upon millions to their deaths as some kind of terrible despot too idiotic and craven to even equip his own men. Reality was his fully equipped army was near Vladivostok near Korea battling Japan. That army in turn double marched...then shot their own officers so they could march 20 hours a day to make it Moscow in time. They were not ordered to do it they wanted to do it.
The day that real army appeared to push the German out from Moscow was the the moment where Nazi Germany would endlessly lose in every operation it mounted. The Nazis were very well equipped and trained, but the Russians were ferocious and utterly relentless display a moral that would have made an ancient spartan week with joy and tremble at the millions upon millions who held it.
There was a reason why the USA was utterly terrified of the Russians in the Eisenhower days.
And yet we here in the US in 2016 even entertain the notion of willingly engaging with Russia in a war. Its like the West learned nothing of going to war with the Slavs.
My point being here no..no I don't think for a second that the Russians would have lost their resolve. The West is clever, sophisticated, effective...the Russians are as tough as fucking tungsten carbide, and even more unbreakable.
Its been joked that Russians are the scary white people. I whole heartedly agree.