Rome — Kingdom, Republic and Empire together — lasted about 1200 years. Rome ruled all over the Mediterranean and large sections of Europe and Southwest Asia. Yet in the end, the difficulties expanded too great while the Western Roman Empire dropped.
Why did the empire break apart?
There is no a unitary explanation, but ended up being rather a number of issues that grew too large to solve.
One issue was that the empire grew too large to govern. The Romans attempted to resolve this by dividing the empire in to the Eastern and Western Roman Empire.
Another problem had been that the Roman men no longer desired to serve in the army. The pay had not been great. Often the legion was not paid at all. Most males couldn't wish to be from their own families and farms for very long intervals. The Romans tried to solve this by employing barbarians to fight for them.
There were other issues. Civil wars broke out in Rome between different factions. The rich grew disinterested in aiding Rome and just wanted more for themselves and their own families. The population of Rome shrank. Trade reduced while taxes and prices increased. The poor were overtaxed. The application of slaves put people unemployed. The roadways dropped into disrepair.
The Romans were aware of these issues and quite often, under a good emperor, they would work on them for a while. In time, the great emperor could be gone and a poor one could simply take their place. Quickly the problems into the Western Roman Empire expanded so excellent your Western (European) Roman kingdom, including the city of Rome, fell to barbarian tribes in Europe.
The Eastern Roman empire continued another 1000 years.