What you are dealing with is the fact that the concept of a "mental illness" is relatively modern (at least in the terms you are expressing it, e.g. PTSD, OCD, etc.). The idea that the mind can have "disorders" of this sort is a relatively "modern" one (19th century or so). This is not to say that there were not asylums and notions about what we would today call mental illness in the past — there were, but they had different ideas about what they were seeing (the behaviors) and what caused them (the etiology — is it a "disease" or a physical "malfunction" or an "imbalance of humours" or a "curse" or a "demon" or something rotten someone ate or an exposure to bad "airs" or what?).

So, let's use PTSD as an example. PTSD as a term didn't show up until the late 1970s. Prior to that there were other terms for long-term psychological reactions to stress, including the famous "shell shock" of World War I. The concept of what we today call PTSD has changed a lot over the 20th century. Let's imagine that we think PTSD as we consider it today is "real" (with many scientific terms, especially psychiatric ones, there is always lots of uncertainty on this point). Does that mean PTSD existed in the past? Indubitably — but nobody would have called it PTSD, or even diagnosed it as being the same thing, because they would have seen it as something else. So if one wants to do a "history of PTSD in the middle ages" you have to take a modern idea/term and superimpose it on the accounts from the far past. This leads to messy results, and leads to you (the present-day person) effectively "diagnosing" a past person based on very limited evidence. (It is hard enough to diagnose someone sitting in front of you who is actually alive, it is very hard to diagnose someone who has been dead for centuries and whose records are necessarily incomplete.)

So you can see why historians are hesitant to do such a thing. For a nice case study of the difficulty of retroactive historical diagnosis, see the case of Charles Darwin's illnesses, which have been a perennial subject of historical speculation.

But let's say we don't think PTSD is a universal human experience. Let's suggest, just for the sake of argument, that it is in some way related to a cultural experience, one that is historically situated in a particular moment (e.g., the 20th century). Could that be true? Sure, maybe. We have other examples of this — for example, Freud and Charcot both studied "hysteria," a sort of fainting that was common among upper-class women in Europe at the end of the 19th century. This is an "illness" that seems to have been at least partially related to social circumstances, an attempt (some have speculated) for women to regain some kind of power in a society that deprived them of power. Or we could suggest, as the psychiatrist Dave Grossman does in On Killing, that the modern experience of PTSD in the military is in part related to the rapidity with which soldiers move between war zones and the home front — to be able to be in Baghdad one moment, and Ohio the next, as we can do in the jet age, causes the brain to incompletely distinguish between these two very different cultural contexts, and causes some kind of backlash or disorder. (I am paraphrasing.) Grossman suggests that PTSD incidence was a lot lower when transportation was slower; if it took weeks to get back to the "homefront," and you spent that time with people who had gone through a similar experience (your fellow soldiers), your brain would have time to process and normalize the trauma you went through.

I'm not, I should say, endorsing any particular psychiatric interpretation. I am not a psychiatrist, psychologist, or what-have-you. I am a historian of science who looks at these kinds of debates. The field of psychiatry/psychology in general has been historically plagued by questions about the "realness" (by which is usually meant, the "ability to transcend cultural context," which may be a self-defeating and inappropriate notion for cultural beings like ourselves) of their mental disorders, and by changing conceptions of the nature of said disorders. Depending on which stance you take for any particular disorder, it will lead you to look for (and perhaps see) different things in the past. Were there eating disorders in the middle ages? Some might say, no: anorexia is a product of a time in which calories are overabundant and culture paradoxically wants us women to look like twigs, or something like that. Some might say, yes: eating disorders might be a result of some kind of "chemical imbalance" (or whatever term/metaphor is in vogue at the time) and some incidence of them would be expected amongst all people at all times. (And they might look at those medieval saints who reportedly lived on nothing but the Eucharist alone in a new light.)