How did the attempts of Justinian 1 to restore the Roman Empire ended?

The Byzantine Emperor Justinian considered his main task to be the restoration of the Roman Empire to its former borders. It is worth recognizing that during his reign this emperor managed to expand the territory of Byzantium almost twice, annexing the coast of Africa, Italy, part of Spain, Sicily, Sardinia. But these territories were devastated either by a prolonged war or by Byzantine tax collectors. The army was very small in number to guard the borders. With the death of Justinian, the empire begins to lose the acquired lands. It should be added that after the death of Justinian, the treasury was devastated by these enterprises.