There have some good articles lately on the indeterminacy of psychiatric definitions, such as "depression:" John Derbyshire's The Anatomy of Melancholy at Alternative Right, Louis Menand's Head Case: Can psychiatry be a science? at The New Yorker, Jonathan Lehrer's Depression's Upside in the NY Times Magazine, and Ethan Watters' The Americanization of Mental Illness in the NYT Mag.

One of the conundrums is that psychiatric terms change over the generations. For example, terms such as "depression" and "Post-Traumatic Stress Disorder" have largely replaced "nervous breakdown" in popular usage. Robert Heinlein sci-fi stories from the middle of the 20th Century frequently feature spaceship captains who suffer "nervous breakdowns" from the strain of command. (Heinlein was a naval officer for seven years, so his space travel stories are really sea stories.)

And, yet, Wikipedia says:

The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a diagnostic system such as the DSM-IV or ICD-9, and are nearly absent from current scientific literature regarding mental illness. ... The closest DSM-IV diagnostic category to nervous breakdown is Adjustment Disorder with Mixed Anxiety and Depressed Mood (Acute).

But few people talk about ADMADM(A) these days. But everybody worried about "nervous breakdowns" in 1942, when Americans didn't exactly have time for things that weren't important.

Moreover, Heinlein's captains weren't suffering Post-Traumatic Stress Disorder because their stress is over the upcoming or current trauma, not necessarily past ones.

And different terms have have somewhat different connotations. Which term you use will tend to influence your thinking.

I noticed this when I was reading up on the Civil War and got to the formidable Gen. William Tecumseh Sherman's psychological collapse in late 1861, in-between his strong performances at the battles of First Bull Run in 1861 and Shiloh in 1862. While organizing behind the lines for the next year's campaigns, he had to be relieved of command so he could recuperate at home. Sherman later joked, "Grant stood by me when I was crazy, and I stood by him when he was drunk, and now we stand by each other."

The Wikipedia page on Sherman uses the old-fashioned term "nervous breakdown" and blames "the concerns of command." In contrast, James M. McPherson's mcpherson Battle Cry of Freedom sometimes uses the more modern term "depression," and at one point suggests that Sherman was depressed by his vision of the logic of Total War.

Or was he suffering Post-Traumatic Stress Disorder over Bull Run?

I certainly don't know enough about Sherman to offer a diagnosis. I just want to point out that you can see how the customary terminology of your time and place would tend to channel your descriptions and explanations.

Similarly, other countries have mental illnesses we don't exactly have, such as running amok in Malaysia (in the 19th Century, every village had to have a long pole with a lasso on the end for snagging the inevitable amok runners).

On the other hand, the power of American culture seems to allow us to infect other cultures with American mental illnesses, such as anorexia. USA! USA!