Other answers get into this, but I'm going expand a little on my comments.
Don't just describe symptoms, explain the reasons for the symptoms. Understanding that there's a problem is way different than understanding why the problem exists.
You have the right idea of not trying to diagnose the MC for your readers. This goes into the writers maxim of "Show, don't tell."
"Eris has PTSD" is overly clinical and explains nearly nothing. Is is because their dog died, her boyfriend abused them, were they in a car accident, did a bear attack them, what?
"Eris mentally curled up into a ball when the stranger entered, who reminded her of her step-father." Better, but what did the step-father do? Was it verbal or physical abuse? Did he hit her or was it sexual abuse? Still, we don't know why.
"When the stranger entered, it reminded Eris of her step-father, who was torn to shreds in front of her as he tried to save her from a wall of shrapnel in The War 10 years ago. He was the best dad she could have wanted and still weeps when she thinks of him." Now we know exactly what's going on: she has PTSD, why it happened (at least partly), what her reaction is, and now we're probably a little scarred now, too. When another stranger walks by 400 pages later and she wipes her eye, we're probably going to remember what's going on.
Most readers don't read a book or even short story in one sitting. I know I don't. I'll forget half the stuff told me in the first half of the story by the time I get near the end. If you leave a lasting impression on me of why things are, I'll remember it much better.
Chris Bunch was really great at this. His book series "The Sten Chronicles" was a great read because of it. I haven't read it in nearly a decade and I've only read it once, but I was just reminded of how the first book started, as I write this. It describes exactly how the MC lived before a Turning Point, what that Turning Point was, how the MC reacted to it, and how the MC changed their life after coming to a Realization. I'm not a writer, so I'd mangle a spoiler if I write one, so I won't. Consider reading at least the first book to understand what I'm talking about.
Again, "show, don't tell" your readers what they need to know.