The Wehrmacht suffered several shortcomings especially since Operation Bagration and the disaster at Falaise. First of all the shortage of manpower meant that most new established and rebuilt army units lacked experienced Officers and NCOs while the need for new units forced the army to shorten its training regimes. Just to give two extreme examples:

• At the Battle of Hürtgen Forest the German Army needed reinforcements so bad that they threw a whole course of an officer cadet school into battle as regular troops .
• The Panzer Lehr Division is considered to be one of the "best" armored units the Germans ever had. But it was formed from experienced personal taken from Tank Schools and Training programs that were now missing to educate new Tankers.

In addition the army received reinforcements personel from the navy and from the airforce that had no experience in Land Warfare (This was the case before 1944 but it intensified with time).

Another shortcoming was the disruption between High Command and Front Leaders. Hitler had the tendency to blame his Commanders for failures, a feeling that intensified after the plot in July 1944. For example when planning the Battle of the Bulge Hitler forbid Model and Rundstedt, the responsible Field Marshals in the area, to make any alteration of the operation because he believed that they would sabotage his plans. Furthermore the Wehrmacht doctrine changed from a flexible defensive with decisive counter attacks (that brought Germany some significant victories at the Eastern Front in 1942/1943 like the third battle of Charkov) to a static defense with any option of flexible reactions being blocked by High Command and even Hitler himself (Plz note that this is of course some kind of simplification but with the end of the war it became increasingly more likely that commanders were forced to sacrifice their troops when they could have been rescued).

In Addition there were also the shortcomings in Equipment. Even though Germany managed to increase its production in the last years of the war it had to deal with a significant decline in quality. Missing raw materials meant for example that high quality steal could not be produced in the required amounts.

Used Sources:

Glantz - When Titans clashed Beevor - Ardennes 1944 Beevor - Berlin 1945

Other answers tried to focus on the German side but the answer lies on the Allied side. The Western Allies landed more than 5 million man and nearly 1 million vehicles in Northwest Europe. Before the Normandy front opened the Wehrmacht had already to divert forces from the Eastern Front to guard the Atlantic Wall and try to contain the Allies in Italy. With the invasion of Normandy this concept pretty much collapsed, before Normandy the Wehrmacht, even with limited ressources, was able to somewhat resist the Red Army. Before the landings Germany "only" used considerable manpower forces to protect the Atlantic Wall, some Panzer Divisions were present but this was connected with refiting this units the influence of the Western Allies on the Eastern Front tank force was still limited but with the beginning of fighting in France, the Wehrmacht was forced do deploy more and more tank units to the West. When Operation Bagration opened and destroyed Army Group "Mitte" more than 2000 tanks were in the West including 600 Panthers and Tiger. Those were missed on the Eastern Front. From here on the Wehrmacht just got overrun and that it took a year to reach Berlin was more the result of incompetence on Allied side than defensive capabilities of the Wehrmacht. The moment Germany had to divert more ressources to West particulary tanks the force ratios on the Eastern Front didn't allow for successful defensive manouvers any longer. This doesn't mean Germany was able to stop advances before but from this point on it was even more difficult. The rapid decline you noticed was the result. Like mentioned earlier, the Wehrmacht was incapable of resisting from mid 1944 but got another year because the Allies, despite what some historians claim, were still not able to just breakthrough and defeat a rag tag army in a short amount of time.

Edit: The idea that shortcommings in training and material and not the arrival of 5 million fresh enemy troops on a new front are the major reason for the Wehrmacht collaps is ...