Kursk was when the Germans lost the strategic initiative, but arguably Stalingrad was the strategic culmination of German fighting power on the Eastern front. The loss of their strongest Army was catastrophic and never made good.
On the Western front I have not seen any scholarly work that suggests an invasion of NW Europe in 1943 was possible, mostly because the logistics would tend to say it wasn't. If the Mediterranean & N Africa theatre offences (Operations Torch, Husky and Avalanche) had not taken place than materiel for an invasion on a reduced scale would have been available, but Torch & Husky are also where the Allies learnt to conduct joint ops & the US Army learnt to fight & blooded its senior command element. Without this experience D-Day is likely to have been a very different tale.
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