Did you see Normandy and the relatively weak German resistance once we landed? Imagine ten times the resistance and the full German airforce opposing us.
Did you see the stats of U.S. war production versus Germany’s? Now imagine all the production capacity not being diverted to Lend-Lease made available to produce even more, and add Britain’s shipments to that, then add the shipping capabilities that wouldn’t have been necessary , then add that available capacity to somewhere in the Med, and instead of one ‘Normandy’ add three or four more at other locations, are just add it to the Normandy invasion, take your pick.
On the German side, you can subtract what was looted from the East, especially food and oil supplies, Germany would be facing a famine by ‘44 at the latest, and subtract the production that utilized slave labor from captured Russian soldiers and other Slav states as well.
You could also add as a possibility Japan wouldn’t have been likely to have declared war if Germany had merely just invaded France and the Netherlands, which would have freed up even more resources for the U.S. and Britain; the Japanese declared war in no small part because of pressure from Hitler; without his successes in driving the Soviets back I would say it is was a safe bet they wouldn’t have succumbed to his pressure to attack the U.S. if Hitler had only France to point to as a measure of his worth as a war ally.
Re a later post asserting having the ‘entire German Air Force’? The U.S. produced some 300,000 aircraft, Britain somewhere near half that; Germany produced a tenth of that. Not much of a challenge to overcome. Multiply most other war production by about the same factor of superiority.