Welcome to the forum!
You are not alone with these feelings and similar situations - it seems to be a repeating theme in many supporters' stories that their relationship with their sufferer within the 3-4 month period was very strong, passionate and loving. Personally I think that has nothing to do with PTSD and everything to do with what is called the 'honeymooning phase' in a relationship, but I could be wrong.
Individuals suffering from PTSD can suffer from a large pool of symptoms, many of these include feelings of hypervigilence (feeling very on edge, anticipating something, never being able to relax), outbursts of anger or extreme anxiety, insomnia and frequent nightmares/night terrors, feelings of guilt and and shame, general mistrust, and general avoidance of anything that can cause stress - whether that stress be good or bad. Some other common ones are isolating from people (especially loved ones) for long periods of time, feeling completely numb, and dissasociation/depersonalization.
It seems to me here that once the initial intense buzz at the beginning of every relationship started to wear off, rather than settling into the strong bond of attachment love that normally happens in several relationships, someone that suffers from PTSD may find themselves struggling to grow attached, or fear growing attached, depending on the nature of their trauma, and their personality as well. Hypervigilence factors into this - growing attached to someone means being vulnerable. It means allowing oneself to be open and accessible. For PTSD sufferers, this is a BIG no no for many. Growing attached leads to the possibility of being hurt - they already have so much trauma to process that the possibility of them being even more hurt is very terrifying for many. (Please don't take my word as the end-all answer though. This is just what I've seemed to recognize in attachment patterns with my own sufferer, and shared experience with others. Your veteran could be different.)
If he requested that you don't contact him, I would respect this decision. Sometimes sufferers can come around once they've taken time to be alone and process their emotions. But I wouldn't hold out on it in case he chooses not to. It's hard, but ultimately, if he asked you to not contact him, there's not much that you can do.
I hope that you take care of yourself friend!