The fact that you've sought out others with PTSD to help you understand and help your boyfriend is a wonderful sign--it's good to know that there are people like you who are willing not only to be patient with the symptoms of PTSD, but want to help enough to investigate, rather than simply distancing yourself out of the personal inconvenience and discomfort those symptoms cause you, yourself.
Yes, it is common for those with PTSD to distance themselves when experiencing difficulties, especially difficulties with their PTSD related symptoms. I do the same-and most PTSD sufferers have that tendency, I think. It's actually one of the hallmark symptoms--isolation, and what is referred to as "numbing", in order to prevent feeling anything, as this feels safer than opening oneself up to feeling the rush of feelings associated with PTSD which threaten to overwhelm us: terror, helplessness, a sense of the world as an impossibly dangerous place that one needs safety from .
Harder to understand for those close to PTSD suffers is the fact that those with PTSD have an even greater tendency to distance themselves from those they are closest to--those who are close tend to be hurt, as you were, or even insulted--to see in terms of their loved one rejecting them, or not caring enough to overcome their symptoms, for their (the loved one's sake). It's hard to understand such feelings, and reactions, when, from your perspective, you only want to help.
But it's important to try to understand what it's like from the PTSD suffers perspective, so as not to be in danger of taking it personally: Think of it like someone not using their thumb at all after hitting it with a hammer. It's highly sensitive to any sensation, any sensation causes pain.
Which is why it's often easier for those with PTSD to interact with relative strangers, than with a loved one--which is often even harder for the loved one to accept, and seems even more as though the sufferer must be rejecting the loved one. "If he doesn't have problems interacting with others, why does he avoid me? That MUST mean he's rejecting me,specifically! If it were just his symptoms, he would be avoiding everyone!"
But this couldn't be farther from the truth. Think about it: Do you have to expose your feelings, to "get close" with relative strangers?
Of course not. They don't expect that depth of interaction. You can just interact with them on a casual, superficial basis.
But what do loved ones expect? They expect something deeper, or they feel insulted that they're not getting it--they experience that as rejection, as "pulling away". It's no surprise that a sufferer would prefer to not be around the loved one at all, and be honest about it being due to his/her symptoms--rather than have to hold the loved one at a distance while interacting with them...something which the loved one is bound to misinterpret as rejection--as the sufferer "pulling away", or "not having the same feelings anymore".
It's most important to understand that PTSD is not something that can "Just be controlled if they really wanted to". It's literally a brain condition, just as though they had a tumor, or suffered a head injury. People wouldn't expect someone like that to just be able to get over their symptoms "if they wanted to badly enough". And the same is true with PTSD.
Only the problems are taken personally by loved ones, as they have to do with feelings and quality of interactions, and with things like hypervigilence, which makes it difficult to go out into the world at all, or interact even with complete strangers, often.
But it's all too easy for the loved one to think "they must just be too weak to "stand up and face their fears like a man", or they're
"frightened of the world", a sign of someone who's immature and avoids challenges, when it's seen in the average person.
But as the sufferer doesn't have an x-ray that shows his brain-tumor, or wear a bandage on his head, as someone with a head-injury would--the loved one is understandably confused by the sufferer's behavior--because as you so insightfully put it-you don't have any frame of reference for it. It's impossible to really get your head around. And it's all too easy when in such a situation to blame the individual for not "having more control".
I'm so glad you've sought out information to help you help him. He's very very fortunate to have someone like you in his life, and I'm so glad that another sufferer has found someone as caring, and dedicated, as well as insightful enough to realize that, as you put it, you haven't had much of any traumatic experiences at your age, much less experiences like those responsible for PTSD.
In my opinion, the best thing you can do to help him is not "take it personally"--to "give him some space" when he needs it, as in this past month. I know it's hard, and you miss him. But being around someone whose PTSD symptoms is actually more likely to ruin a relationship in a lot of ways--as I mentioned, it's almost impossible not to take them personally when you're there, and they seem directed at you. But it's hard for the sufferer, as well, to know that they've not only reacted in ways they regret to someone they honestly love--and men especially, to have the sense of pride they need to retain self-respect--when someone they love (especially a girlfriend/spouse) has seen them at their weakest. It's even harder to hold your head up after that (probably even moreso for your boyfriend, with his Marine Corps background).
A great book to read to learn the potential severity, and approaches to PTSD is "Trauma and Recovery", by Judith Herman M.D.
I know it's difficult being with someone who has PTSD, from personal experience. Remember that the worst, most unrealistic, and harmful thing you can do is take it personally--harmful both for you and for him, as well--and that even though the wounds can't be seen on an x-ray, or by a head bandage, they're just as real, just as physical (relating to neurophysiology), and just as important to seek treatment for.
Because there is treatment, and where there is treatment, there is always hope. There are a number of trauma-related therapies: EMDR, Harm Reduction therapy, exposure therapy, Dialectic Behavior Therapy (DBT), and others. And even though you miss him, understandably, among the best things you can do are understanding that he has a real injury that isn't his fault, and not taking it personally when he tells you that his symptoms are "acting up", remaining supportive, and encouraging him to seek therapy, as well as reading about PTSD yourself.
So glad he found you, and glad you're helping him help himself. There are probably support groups for loved ones of those with PTSD, where you might be able to get even better information for your specific situation and feelings, as well as support from others in the same situation during times of stress and confusion.
Good luck!
