I hear many things that all empires collapse inwards, it's too difficult to govern such a large area, too many cultures.
Perhaps it is the conflict, Nazi Germany for instance couldn't proceed invasions because of the enemy force.
So I want to know why older empires such as the roman empire fell and how empires that began in the 1800 (British etc) ended up falling apart.
Really every empire is different, there is no one cause that is visible in every empire, some similarities may exist, but in my opinion there is no one cause for an empire fall.
Take the Roman Empire and british empire. The Roman Empire fell because of constant civil wars and strife, mismanagement, constant encroachment from outside enemies and a series of inept rulers.
The British empire, didn't really suffer from civil war, and it wasn't being attacked by outside barbarians. Some similarities may exist but as you can see there every empire is different and so are the reasons the various empires fall.
As /u/Ambarenya says, unhappy empires are all unhappy in their own way. However, Ian Morris in Why the West Rules postulated that empires (or more precisely social development spheres) collapse under the weight of their own character. With the Romans, for example, this can be seen in the way that the vibrant commercial networks across the frontier, as well as the practice to hiring barbarian mercenaries, led to the development of the sort of organized political societies that would later directly cause the collapse of the Empire.