Germany didn't have the manpower or the economy for a protracted war. Hence the early reliance on "Blitzkreig" when taking France/Poland/etc...
The moment that first push petered out and it became a slog instead of a blitz, they had already lost. It was just a matter of time.
As someone already mentioned, the Nazi state's entire strategy relied on getting access to Russia's oil reserves before their own ran out. They were on a ticking clock until they could make that happen (which ultimately they didn't).
As for operation Sea Lion giving them British Empire oil, why would you think that taking the island of Britain meant that they automatically take all the colonies of Britain? Conquering an island doesn't mean they get Canada and India (for example). That's not how war works.
You'd think they would have learned from the last time they tried that...