PTSD has two sides of lonely.
The first is when you're experiencing symptoms on a daily basis, and you look for birds of a feather to flock together with-- you sit at the bar of life and get drunk with other people who have the same symptoms as you. It doesn't make you feel any better, because they all have their own problems and don't have any extra resources to help you. But it's an environment you know, and it's always there, and so you keep coming back to the bar.
The second is when you've pulled your head out of your ass and got some help that really did some good-- you've got professional treatment and although you need more practice, you can now cope. You realize that a bad day is a bad day, and you learn to admit to yourself that even though something wrong today, you probably won't die as a result. You'll find a way to get through it, and tomorrow or the next day will be better. You have tools. You value yourself. You have things to prove, but proving that you are strong and are a survivor is already covered.
The second kind of lonely is the most difficult.
Once your toolbox is full of tools for coping-- once you've faced your trauma, and beat it down with a stick, and can manage-- then you're someone different. You're better. Not better than anyone else, you're the same physical person, but you're much better at being you. You've changed, and that's a very good thing.
The world around you hasn't changed.
Those drinking buddies? They're still where they were yesterday, except they're no longer funny. You've crossed over to the other side, friend. Now you need new friends, people who have their act at least a little bit together. This is a new place for you, or at least a place you haven't seen much of recently (because you've been hard at work healing), and it's scary and unfamiliar.
And, right now, it's lonely. You've had experiences that most other people have not. You've been places most other people only read about or see on TV. And most important, you have tools for dealing with life that most people don't have (and that many people never will have).
And, it feels new, and a little scary, and you (I) have to set about making this new world into something that feels like home.