You don't need me to help you google "reasons why the roman empire fell", but apart from apathy, other reasons were: economically and militarily they were a wreck, especially by the end of their respective Empires, and although the Byzantine did well on their own, they didn't work together with the Western brothers for resources or to repel outside attacks; which in turn was another reason, they were invaded by the Huns and other tribes, so much that Rome was attacked a total of 5 while controlled by either the Western or the Byzantine (2 under the Western, 3 under the Byzantine Empire, and another one more but it was guarded by Papal States), this obviously demoralized their people; mass migration was another reason, mixing two different cultures didn't bode well for them, only helped in their disunity and infighting; the rise of Christianity as a new faith divided the people; the government was corrupt, but what else is new; the Empire was so big they couldn't control it, it was a logistical nightmare.

All this would be better explained if it was explained by a professional, in a documentary, or both. I know of two. One called "Ancient Rome: The Rise and Fall of an Empire" made by the BBC in 2006 with 6 episodes of 1 hour, and "Rome: Rise and Fall of an Empire" made by History Channel in 2008 with 13 episodes of 43~ minutes. The names are alike but they're 2 different docs, I remember having watched them a while ago. At the moment I'm trying to find them for you but I only found the History one: https://www.youtube.com/playlist?list=PLskL8uu7zHrBXI5LEQIVMCAsq7LKpCgOC. Should be enough I think.