I've at minimum skimmed all the comments so if im repeating, sorry!
First of all, no, you're not alone AT ALL. Second, your friends without PTSD in their lives will not understand. Save yourself the trouble of doubting your decisions and trying to explain away things they won't understand.....it makes it more stressful for you when generally the idea is stress relief through sharing (this took me too long to figure out). Third, if you are truly committed to him (and its ok if you're not, nobody here is judging) get used to this right here. There will forevermore be periods of isolation and you don't know when they will come or how long they will last. During these times, you are essentially single. Your social plans, you'll be showing up alone. The sink is backing up? You're getting quotes from plumbers....and you're not venting to him about how they showed up 45 minutes late and got mud on the carpet. Dog is hyper? You're walking him. Infant screaming her head off? You're rocking her back to sleep. So, while he's setting himself up for the future, set yourself up for the future. As various thoughts arise that you want to text or call him about....find another person or activity to fill that void. My mom usually gets my calls and texts about my random small victories ("got the whole house clean before yoga! Whoop!") And my various girlfriends get the small losses ("I can't believe that weasel turned me in for having my shirt untucked after I clocked out!"). On particularly complain-y days meditation to the rescue! Other things to expect: you'll feel more alone when he's at home with you isolating than you do right now away from him. When you're in the mood *wink* you'll get rejected (if you even think trying is worth it). If he's oddly in the mood you'll reject him because your emotional needs are not being met. You'll wake up from his nightmares before he does and watch him thrash around with a look on his face you never want to see on your man. And you'll want to wake him up but you'll be scared of accidently getting punched and you won't want him to be up the rest of the night AGAIN because he JUST fell asleep. You'll feel like its not worth it and you'll have to remind yourself daily why it IS worth it (I suggest writing a list of all the things you love about him when things are good). Any big talks or big decisions will be put off til later....if its time senstive its either not happening or you're deciding on your own.
I know that my sufferer tries to shield me from his "dark" mind (his word, not mine). He has told me that he intentionally keeps some PTSD-related things away from me. He says it's because I have no frame of reference and can't relate. I think its also in part to preserbe my innocence. I got that same vibe from your vet in your original post (my sufferer is also a combat vet with a traumatic childhood).
Anyways, these are some things I wish I knew sooner. I don't know if it would have changed my decision to date him. But it would have at least made figuring out my own coping strategies easier/faster. I also suggest writing down questions with specific examples during this period. During isolation, I always have questions about what I can do better as a supporter, or what the heck his behavior was about on that one day. If theyre written down you can ask when things are good for him and you wont forget in the meantime. Having conversations with my sufferer when hes good about when hes bad has been the most eye opening and has helped A LOT with my mental health during his isolation periods. His most recent isolation period was about a month. I got through about half of it before I cried (I'm a crier....that was a victory!). And it was when I was sick and tired of missing him, feeling like his roommate, and receiving damn near zero affection....i decided to give him a kiss goodbye since he actually acknowledged me first that morning. I went in for a peck and his eyes didn't move from the TV and his lips did nothing. It was like I wasn't even there. I said bye, told him to have a good day and as soon as I reached the safety of my car, it was an ugly cry. So ugly of a cry i couldn't drive. Pulled over after I left the driveway.
Ok....ill stop rambling now. Sorry.