IMHO, Germany lost the war the day they invaded Poland and Britain didn't stay neutral. The German economy didn't get into full scale war production until 1942. The German navy didn't feel they would be ready for a war until 1945 with a minimum 300 U-Boats (100 on station, 100 in transit and 100 in port at any given time).
With an unsinkable aircraft carrier so close to mainland Europe, backed by an 'everything short of war' America Germany didn't have the resources or industrial capacity to compete. A war with Russia would be the last thing the Reich needed. I don't see how they could have defeated Russia, too big, too much distance to cover for an army that existed on boot leather and horseshoes and too many men to fight.
And the Russians learned. It took them several years and millions of men but they got better tactically and strategically.
Napoleon said of his foes at Leipzig in 1813 'These dogs have learned something.' Even after the war no German general admitted that the allies got the better of them. It was always Hitler's fault and interference