The deuteragonist of my trilogy, The Ragnarǫk Sequence is heavily implied to be Jeanne d'Arc (whose characterisation takes many cues from Artoria Pendragon) and serves as the protagonist's moral compass, criticising him for his less than heroic acts.
The series suggests that the deuteragonist is even more messed up than the protagonist and has Major Depressive Disorder, but hides it better (which says a lot, given that the protagonist suffers from severe cases of Borderline Personality Disorder, Body Dysmorphic Disorder and sex addiction). Such mental instability stems from the trauma of the Hundred Years War, struggling to conform to modern society's values where religion is outright mocked, everyone she knows being dead and having no human contact for over six centuries.
Things get worse when the deuteragonist learns that the various visions of Angels and Saints she had in life were attempts by an aeons-old, serpentine monstrosity with omnicidal tendencies called Jǫrmungandr to manipulate her into freeing it. This revelation is a crucial factor in making the deuteragonist abandon her religious views, as he comes to believe that she died for a lie and another is when Jǫrmungandr's "father"/"messager", Loki reveals that he killed and masqueraded as a priest giving her emotional support to destroy her sanity and render the protagonist susceptible to influence from Jǫrmungandr.
All of this insanity ends with the deuteragonist masturbating to a memory of the protagonist having sex with his former love interest, showing that she has hit rock bottom and leaving her wanting to commit suicide. And yes, it's as bad as it sounds.
The problem is that I want to write in a manner where the deuteragonist's sanity slippage occurs in a gradual and believable fashion, rather than some contrived, implausible way.
How should I accomplish this?