This monologue piece is is taken from an anime (魔法少女まどか☆マギカ, episode 2, this starts at ~10:55). It makes sense to me except for the highlighted part:
Audio extract. [I've included the earlier bits for context as well]
The way I've seen Xしか～ない used is normally (always?) to stress that there is nothing but X.
In this sense, it goes something like:
Us, who have haven't found it [the wish], know nothing but the unhappiness of that degree [???]. This makes little sense within the rest of the monologue that stresses how much of "happy fools" she and her friend are [context: they are granted a free wish in exchange for becoming magical girls but can't think of anything worth risking their lives for].
This makes me think I've fundamentally misunderstood this part of grammar. Is that so, and where did I go wrong?