* If all true, my concern with this dynamic is two-fold. The first is that (a) its just relegated to background color (no mechanical engagement that interestingly impacts the player's experience of their decision-space) and (b) , following from and relating to (a), only the GM may be in on the secret because the nature of the color is obfuscated to such a degree that both the experience of the PTSD and the impact of the reveal on the player is muted or isn't as potent as hoped for.
Now (b) following from and relating to (a) above is an outgrowth of a few things; (i) the autobiographical cognitive space of the participants (how does various forms of technique and overall play impact the idiosyncratic immersion of the individual participants) and (ii) the GM's ability to
do the thing (which includes
a lot of little things that need to add up to
the overall experience and finality of the thing).
* Here is how I might consider easing the tension of the bolded/italicized above if I was running 5e. Are you familiar with the Abyssal Corruption, Beast Transformation, and Vile Transformation mechanics in the 5e DMG? You might consider an iteration of them coupled with the Paint the Scene process of The Between. As follows:
1) Player rolls Charisma Saving Throw after conflict. On failure, the GM gives the player a new Flaw or Trait which clearly depicts the state of the character (the thematics, the mental duress, et al) and invites the player to describe the PC's cognitive orientation to the event based on that new Flaw or Trait.
2) GM can either veil the mental duress/illness Transformation mechanics or share them with the player (my guess is you would be veiling them).
3) At the next Long Rest (or whatever appropriate time), the player makes another Charisma Saving Throw (perhaps this is progressive or perhaps its the same DC as the initial). On a failure, the PC gains 1 level of Exhaustion. The GM briefly describes an object or an event in the imagined space (a flower that seems to reach to bees but never gets attention while other flowers do, an inkeep looking forlorn as they absent-mindedly mop the same area of floor to no affect for minutes) and invites the player of the character to vignette their character's thoughts on this object/event and relate it back to the events of (1).
4) Same deal as (3) except a final failed Saving Throw cements the thematic and mechanical situation which can only be resolved via Remove Curse or some mechanical + fictional process (I'm thinking
Paladin's Quest move or
Wizard's Ritual move in DW or Make a Plan in Stonetop where the player and GM "ask questions and use the answers" to come up with requirements/conflict resolution Win Cons and related fiction to resolve the dramatic need).
So here, if you (a) veil the overall situation/journey of the condition but (b) involve the player thematically while (c) introducing decision-space & gamestate altering consequences, you might get a result that both lets everyone experience the reveal (you included as the player is letting you in on their character's cognitive state and attendant interpretation of thematic touchstones) and it might be impactful to the player of the character (either the same but in a different way or perhaps even more impactful).
So those are some thoughts on your conundrum (as it pertains to what I wrote upthread) that lands nicely in the "why do RPGs have rules" area! Do with them as you will (including drop them in the dustbin asap!)!