While I´m speaking from the position of a layman, from what I understand Medieval Germany/The HRE, as the middle ages went by, slowly broke apart internally until it entered the Early Modern period a barely functional collection of dozens of different polities, only being reunified in the mid-19th century. Which is jarring, since other medieval ``countries´´ , Like England and France, mostly became more centralized and nationalistic during this period.
Of course this could be a misconception. But I would really like to hear about it if it is, as well as the origins of the misconception.
When is this "collapse" supposed to have occurred? Historians have often joked with a standard quip that the Holy Roman Empire of the German Nation was neither holy, nor Roman, nor entirely German, nor a real Empire. In fact each of these terms in its title is disputable. What is indisputable, however, is that legally, it existed from either counting Charlemagne's coronation (800 AD) or at least Otto the Great's (962) as the starting point, to its demise in 1806, when Habsburg Emperor Francis II dissolves it after the Battle of Austerlitz, which gives the HRE the undisputed stature of being the longest run legal entity in European history after the Catholic church itself.
While there were periods of disaster within the Empire that rendered it merely paper status at times (such as during the interregnum or the Thirty Years War), its existence and continuation was never in question during this period: ever major political participant in the long history of its internal conflicts aimed at its continuation, or change of its placement within, but there was never a drive to abolish it whole. (And such disasters were plenty in the long run of the Kingdoms of England and France as well).
There is a I believe misunderstood trend in modern historiography to compare the HRE's complexity to the more centralized kingdoms of France and England, letting readers often walk away with the notion that the Holy Roman Empire "did not work" when compared to, say, absolutist France; but if it did not function well, why did its functions and institutions (e.g. the Reichstag congregations, the traditional crowning of the German King in Frankfurt, the Order of the Golden Vleece, the principles of Imperial law, etc) last that long? The Occam's razor answer is: they lasted because they actually did mean something.
The HR empire was never, in a way, the kind of empire you could compare to the British Empire of the 19th century, or to the German Kaiserreich of the late 19th century, or even the original Roman Empire: The Holy Roman Emperor was never a dictatorial ruler who could simply declare laws that were followed to the bottom of a hierarchy. It was, at all times, a complicated political dance with a plethora of centrifugal forces of prince electors, powerful bishops, mercantile city alliances and powerful neighbors. Only, perhaps, Charlemagne wielded the kind of power imagined to be that of a Roman Emperor, yet even his political constructs rested on internal alliances and compromises the kind a late Roman Emperor of the ancient world usually did not have to make.
If anything, the Holy Roman Empire granted its subjects a canvas onto which they could paint their collective identities, within the legal framework of a larger, Christian historical understanding of the occident, as artificially constructed as it was. That is, I think, one of the main reasons it lasted so long: one could very well be a subject of the Holy Roman Empire and, at the same time, be a patriotic citizen of one's own suzerainty (such as the Kantons of the Swiss confederacy), without the two being an intellectual contradiction.