Norwich's history, which is quite well written, chronicles the empire from its founding by Constantine to its last feeble flickers in 1453. In some ways it is hard to fathom how an empire which at one time stretched from Italy throughout Eastern Europe and North Africa and Levant had shrunk to little more than a rump in Europe just before its fall. And Norwich covers it well, going emperor by emperor until the end (I assume - I am still just before the Fourth Crusade in 1204).
Not everything is the result of bad decisions - after all, outside events effect all of us that we did not initiate and did not control. But like individuals, empires and states are also victims of their own self-inflicted wounds. That I can tell to this point, there were three main ones:
1) Internal Conflict - The Byzantine Empire, it seems was rife with rebellion and infighting, almost from the death of Constantine. These internal rebellions inevitably weakened the state: beyond the cost of money and politicization of sides (which often ran to violence), the internal rebellion spent the lives of the Empire's soldier's not defending itself but rather fighting with itself. Those soldiers and the lands they were recruited from (and was lost) were missed long after the rebellions themselves had died.
2) A Lack of Sound Fiscal Policy - The Byzantine Empire, like many other empires and states, had trouble managing it funds consistently. Under some emperors fiscal prudence was maintained, under others all control went out the door. And the military, which defended the Empire, was caught in the issue of either not being funded enough or trying to make up for poor funding (which over time necessitated higher and higher taxes). It is not that they were alone in this as a medieval state - autocrats are notoriously big spenders and do not care where the money comes from - but it did ultimately create a state where no money could be spent at all without borrowing it - and all that this implies.
3) Internal Division: While the Church and the State were theoretically split in the Byzantine Empire, both the Church and the State meddled with each other. From the State's side, it took sides in the religious conflicts of the day (the one in question was of Icons) - and pushed the opposing side away. Likewise, the state acted to enforce religious purity (Bogomil and Catharism). What did all of this do? Embitter and turn away citizens and future citizens from supporting the government at the very times it needed it
Ultimately the Byzantine Empire ends with a sad sort of silent "pop", hardly befitting its history. In the end there were few to fight for her and her reduced size and scope made here as dependent on others to fight for her as herself.
Any and all resemblances to any current nation-states is purely coincidental...