Outside of the general proclamations on the "Rome-Berlin Axis" around which European politics - or the world, if we want to be overdramatic - were supposed to rotate, the Italians had major concerns for the expansion plans of Germany.

To understand this though, one needs to keep in mind - and I suppose many readers are used to the concept of a more "strategic", semi permanent alliance such as the one between the US and the UK - that alliances between nations are often based on mutating circumstances and interests. So that the French relations with Russia, or the Austrian relations with Prussia and later Germany could change significantly during the XIX Century, without that representing any form of treason.

Around the time of WW2, the influence of popular opinion was already strong enough to make international relations a bit more "personal" and less practical, but Mussolini's approach was in this sense quite practical, in a late XIX Century acception.

While a certain current of the Italian political and diplomatic field had certainly stigmatized the behavior of the Allies towards Italy at the end of the Great War, this indignation - especially when coming from the nationalist field - was hardly predicated on a genuine feeling of betrayal. Rather it was the confirmation of their general view of international relations, which represented a very close analogue of a naturalistic, "natural selection" approach.

As the relations between states were always, inherently and structurally conflictual - driven by the objective factors of population increase and need to secure resources - any nation pursuing her national interests was only doing what was natural and demanded of her. Going against those interests was a perfect way to ensure the nation's decline and eventual death (see for instance nationalist F. Coppola's contemporary Italy in the Mediterranean - July 1923). As Italy's interests laid mostly within the Meditarranean, these objective factors were bound to direct her action against France and Great Britain, and therefore to seek the alliance of Germany (obviously there were those who disagreed on this general view, but at least we have a diplomatic rationale for the alliance with Germany).

Mussolini didn't subscribe entirely to the most "naturalistic" view of the nationalists, as he acknowledged the decisive importance of "ideal" factors, so that elements of loyalty and trust in international relations were highlighted for the public and traditionally - and especially later on - came to represent a central element of the Fascist propaganda of war.

I would recommend checking out these two answers for further details on the specific form of Italy's concerns: here with additional replies by /u/Aleksx000 and here.

With that said, the apparent rapid increase of German power in Europe and the similarly apparent inability of GB and France to oppose it confirmed the general view favorable to a consolidation of the Italian and German relations. Here both Mussolini's confidence in a German victory (in the inevitable European conflict to come) and his concerns for a German affirmation worked in the same direction: Italy should seek her affirmation by playing a decisive enough role in the game, or at least by taking the winning side, unless she wanted to be reduced to a second rank power.

And this was also the idea behind the unfortunate "parallel war" attempts, where Italy choose to pursue her own Mediterranean objectives after the realization that the conflict was likely to last longer than originally supposed, in an effort to maintain her relative stature of great power compared to the German Reich.

As for whether Mussolini trusted Hitler on a personal level, the truth is that Mussolini trusted very few people, and his relations with Hitler weren't really a matter of trusting Germany - notably though, his relations with Hitler were and remained somewhat cordial even when the war turned to the worse, and Mussolini was somewhat moved by Hitler's loyalty to their alliance. Even if the thing mattered nothing in the great scheme of things and he certainly had no delusion on Italy retaining any substantial international independence after 1943. In immediate terms though, Italy was never looking forward to (and never looking at) the perspective of an absolute affirmation of Germany; rather Italy hoped for a negotiated peace that left Germany in control of central Europe, diminished the French and removed the British from the Mediterranean. In this scenario, there is no German betrayal, because the fundamental directions of German expansion are yet to be accomplished.