(Sarcastic Extra: How can the entirety of the history of the Italian peninsula, even to this day-like as we speak- be basically a big question mark "how problem/chaotic thing X began at all and how could such an unsustainable thing sustain itself for so long?)
Mussolini's dictatorship was never total, nor was it particularly consistent. This great answer by u/Klesk_vs_Xaero is a great overview of Italy's descent into dictatorship, and the fluid and evolving dynamics between members of Mussolini's govornment.
I also wrote this shorter answer about Mussolini's relationship with the Italian monarchy. The King's institutional role had remained intact over the course the Fascist Ventennium: it was still the monarch who appointed the head of government. After Italy's defeat in North Africa, King Victor Emmanuel removed Mussolini as head of government, appointing in his place General Pietro Badoglio. The King and the new government headed by Badoglio signed an armistice with the Allies, and faced with the prospect of an imminent German retaliation fled to Brindisi, in Southern Italy. Mussolini, initially subject to arrest, was sprung from detention by the German Special Forces and propped at the head of a puppet state in the North. Italy was effectively split in half, and would remain so until Mussolini's assassination and the end of the war.
Added to this was the anti-German insurgency. Prior to Mussilini's rise, Italians were already in a state of near-endemic revolt; Communist paramilitary activity was so widespread that the years 1919 and 1920 in Italy are called the "Red Biennium" (I also happen to have written about that and how it led to Mussolini's rise). Although the Fascist state was able to put a lid on civil strife, conflict again erupted in the face of the German occupation and Mussolini's hapless puppet-state.
Now, with regards to your elaboration on "the entirety of Italian history;" I'd just say that in reality, the post-war government of Italy was rather stable. Although the communist-aligned paramilitaries who ultimately assassinated Mussolini and brought down his government would go on to become the influential Italian Communist Party, every single election between 1947 and 1982 resulted in a government led by the very centrist and bland Christian Democratic Party. Although individual politicians might come and go, and single coalitions might vary, many post-war commentators in Italy called the country a "partitocracy," run by a limited number of political institutions. And when the Christian Democratic Party's hegemony was broken, the Socialist leader Bettino Craxi would go on to become the longest-serving Prime Minister Italy had seen since the dictatorship! I actually wrote about that period of Italian history here, through the lens of the Italian Communist Party's political strategy.
However, if you're referring to Italy's historic political fragmentation, its roots run much deeper. That's actually my flair area (and my favorite part of Italian history). I wrote about that here.
Today's my turn in the kitchen; so I'll link you this previous answer of mine discussing "resistance" with a specific focus on the 1940-43 period.
Considering a more genral time span, you are probably thinking of the Civil War that begun in September 1943 when the King's Government in the lands occupied by the Allies signed an unconditional surrender and the Nazis established a puppet Republic led by Mussolini in the North. A Civil War that lasted until the liberation of northern Italy in April 1945.
Before that point, the amount of consensus, especially that of Mussolini himself, was significant; so that after the dissolution of the Socialist Party in 1926 no real internal opposition remained - and even in the 1922-26 any opposition was minoritarian.
If you have more questions, I'll answer in more detail once I am done cooking!