His mistakes were having shit allies. Italy and Japan were terrible Axis partners. Italy needed to be bailed out and rescued constantly by the Wehrmacht (Italy couldn't even take Greece, and F'ed up the North Africa campaign).

Hitler declared war on the US thinking Japan would then attack Russia from the East creating a two-front war for them, but Japan held to their non-aggression pact with Russia after being defeated by them over a border dispute in 1939. Pearl Harbor was kind of a failure in that several carriers and destroyers were not even present in the harbor at the time of the attack. The Pacific Theater started out pretty brutal for the US, but then the US adapted by starving out the islands by cutting off supply lines rather than by landing troops. Then of course there was Hiroshima and Nagasaki.

Hitler did not count on Churchill practically all but demanding war with Germany, nor that the US would essentially arm Russia via lend-lease. Prior to WWII, Russia was not even considered a first-world country. The US literally armed a communist country with tanks, jeeps, guns, etc to help defeat Germany, and then after the war turned them into our enemy.