Leah Libresco is stymied at this week's edition of her Pope Francis bookclub. (You won't be able to follow the rest of this without reading that post.)
I'm not guaranteeing for my own interpretation here, but I think the problem is a scope error.
Leah seems to think of the general project of moral discernment, but right here Bergoglio seems to be talking about a particular sub-project. Going by the first page, the intent of the whole chapter is pre-discerning typical temptations affecting the making of "the various apostolic decisions we must make in our pastoral work/efficacy". (Or something like that, I'm going by the German edition here.) So it's not just discernment in general, it's discernment on evangelizing and in particular evangelizing the culture.
In that context, the struggle at hand is not the struggle for every individual soul but the cosmic one, which in fact is already won. Or more precisely it is ongoing but we know the enemy is fighting in an irrecoverably lost position even if it looks like he's winning a lot of the time.
The pessimism Bergolglio talks about would then basically boil down to loosing sight of that victory and reducing the struggle to what it sometimes looks like.
For example, from the inside compromising eternal goods for temporal ones often looks like making the sacrifices necessary for winning the battle. An example of the example would be identifying one's Christianity with a political side and then treating every election as a bonsai eschaton. Looking at it this way, the following section on separating wheat and chaff before the time looks very connected.
Now I'll go on to blatantly substitute my reflection for Leah's:
One of the underlying ideas here is that, once one has made a fairly stable decision for good, evil will mostly present dressed up as good, or, to say the same in Jesuit, sub angelo lucis. But then it needs a costume appropriate to the context. So basically examining different contexts and thinking of how evil may look there is a heuristic for figuring out costumes evil may be wearing. This is somewhat similar to the modernist project of cataloging fallacies and biases, just for morality rather than rationality in general.
Summarizing in a very "anacultural" way, I think this chapter is mainly about evil dressing up as practical rationality. Bergoglio has several examples of how it may do so: By the pessimism route (pessimism seem like realism and realism is rational), or by urging premature separation of wheat and chaff (categorizing feels super-rational), or, somewhat obviously, by general sterile over-intellectualization, or by slacking in petitionary prayer (petitonary prayer often seems impractical).
And he categorizes all of this as failures of faith, because they separate a theoretical faith from practical efficacy. It's interesting to translate this to my reframing: When evil dresses up as practical rationality, it's actually trying to separate our epistemic and practical rationalities.
By the way, my spell-checker wants to correct eschaton to Charleston. I suspect this would be super-funny if I ever had been there.