Well, but the total collapse of the eastern front Summer 1944 was bec of Hitler was more feared of the invasion than of the rushing Red Army.
At Normandy the German Army (strong potential) was bleeding out slowly due to constant Air attacks and extreme Artillery fire (the so called "doctrines" of the US-Army of fire superiority, they didn't focus much on infantry, tanks). But still held the front 2 months. Now think about these forces on the eastern front at "Bagration"....... instead an infantry division had to cover a huge frontline 
In a Russian-German only war no side would be able to make a major breaktrough the next years.
Gen Guderians opinion ....
Well the thing was, that Hitler knew that the Allies would attack (they fooled him with this but that is another story) so he thought he could first fight off the Allies in the West and then get back to the Eastern Front where he believed his troops would do everything they can to stop a Red Army (that was supplied better and outnumbered the German Troops) to hold them off till the Western Allies are defeated. What happened then is history
