The Germans were going in reverse well before the Western Front got started. Also, until shortly before D-Day almost all the German units on the Western Fron were second and third rate units, with the elite units facing the Commies.
Almost 90% of the German soldiers who died during WWII were killed by the Red Army.
Once the Soviets recovered from the shock and surprise of Barbarossa, the Germans were no longer able to roll over them. And, as numerous attackers (Charles XII, Napoleon, Hitler) have discovered, if you don't beat the Russians quickly, you won't beat them at all.
I was talking about WWI where Germany was kicking the crap out of Russia but pulled back to send troops to the western front. If Germany had kept up the pressure Lenin would have never got back to Russia and the there would have been no October Revolution and no Red Army.
If the US had not entered WWII and helped supply the USSR, the USSR would have repulsed Germany but not have been able to finish Germany off.