Germany had a war going on with the Soviets at the same time you know.
The US provided massive amounts of aid to the Soviets, British, and Free French. The US also was prosecuting a full-scale war in the Pacific. All the while, the US economy never went to a full war mobilization (it topped out at 50% mobilization).
Germany did bite off more than it could chew, but it wasn't invading Russia. It was carrying out actions that resulted in the US getting involved. After that happened, it was largely a done deal.