Ever since they first met, Cassian has done nothing but belittle and insult Nesta. In the Ember and Wings bonus chapter, Cassian says that Nesta wields the words and the coldness as armor to keep everyone from seeing who she failed and how she didn't care until it was too late, and how she's a spoiled girl. Which, yes, a lot of SJM's couples start off looking down on or insulting each other, but if they're endgame, they usually come to see how amazing the other is, and they admire each other, and trust each other. Cassian still doesn't seem to think much of Nesta. In HOFAS, he doesn't stick up for her or protect her from Rhysand threatening punishment or possible execution. Which tells me that he still views her as this spoiled girl, who just failed all of Prythian.
No wonder Nesta hardly ever opens up to Cassian about her trauma. I wouldn't open up to someone who constantly belittled or insulted me either. Can you imagine how Cassian's going to feel if he ever finds out that his "spoiled" mate was abused for years as a young girl? When he finds out that her parents failed to protect her? That the reason she's so amazing at dancing is because of the abuse she suffered for "failing" at it?
Can you imagine finding out that some man laid hands on Nesta, and then going ahead and calling her a spoiled failure?
A spoiled girl wouldn't have been able to survive being boiled alive and violated in the Cauldron.
A spoiled girl wouldn't have been able to survive being sexually assaulted (yet again), almost raped (yet again), and almost eaten by a kelpie.
A spoiled girl wouldn't have been able to survive the Blood Rite, where she was almost raped yet again, multiple times, let alone become Oristian.
A spoiled girl wouldn't have sacrificed her powers to save other people's lives.
A spoiled girl who doesn't care about others wouldn't have given Bryce the Mask to defeat the Asteri and free Midgard.
Cassian has failed Nesta far more often then she's ever "failed" others, but yet he doesn't seem to care...until it's too late, anyways.