Hmm, here is the definition of "understand" from dictionary.com:
un·der·stand http://cache.lexico.com/dictionary/graphics/luna/thinsp.png
1.to perceive the meaning of; grasp the idea of; comprehend: to understand Spanish; I didn't understand your question.
2.to be thoroughly familiar with; apprehend clearly the character, nature, or subtleties of: to understand a trade.
3.to assign a meaning to; interpret: He understood her suggestion as a complaint.
4.to grasp the significance, implications, or importance of: He does not understand responsibility.
5.to regard as firmly communicated; take as agreed or settled: I understand that you will repay this loan in 30 days.
6.to learn or hear: I understand that you are going out of town.
7.to accept as true; believe: I understand that you are trying to be truthful, but you are wrong.
8.to construe in a particular way: You are to understand the phrase literally.
9.to supply mentally (something that is not expressed). –verb (used without object)
10.to perceive what is meant; grasp the information conveyed: She told them about it in simple words, hoping they would understand.
11.to accept tolerantly or sympathetically: If you can't do it, I'll understand.
12.to have knowledge or background, as on a particular subject: He understands about boats.
13.to have a systematic interpretation or rationale, as in a field or area of knowledge: He can repeat every rule in the book, but he just doesn't understand.
Nowhere does it imply that one must experience to understand. In fact, the definitions seem to communicate that the definition of (to) "understand" means that one learns, acknowledges, accepts, all without having to actually have an experience - to be "open" to the facts or ideas.
I think by believing that everyone who doesn't have PTSD will not understand us is very negative thinking and only furthers the idea that we are isolated from "others".