The second the Allies invaded France, that was the beginning of the end for him. He was stupid to think that Germany could fight 2 fronts. Russia was already holding the Germans back due to the millions of people in the Russian army. And the Russian winter stopped the German troops from advancing to Moscow.
To the left of Germany, you had the other Allies. America at the time had the biggest economy and had PLENTY of resources. The British Empire was too big for Hitler to fully invade, so he tried to dampen the 'souls' of British citizens by bombing them all the time. This didn't work either.
So, the British Empire also had thousands of men in their army along with resources from Canada etc.
So by the time the Allies invaded from France and Russia was pushing back, Hitler's army was dwindling in numbers, didn't have any support from Japan since they were fighting the Americans. Their resources were dwindling and once Mussolini was overthrown and the allies took over, Hitler again had to move troops to fight them too.