Ok first of all, what is PTSD? Post Traumatic Stress Disorder was first discovered during WW1 and was called "shell shock", "war neurosis" etc. It was basically a failure to recover after experiencing or witnessing a terrifying event.
What do writers get wrong?
Writing a character with PTSD is great for a character arc, if you can pull it off. Otherwise, it can come off sounding melodramatic.
First thing to do is research more. There are more symptoms of PTSD than flashbacks. Flashbacks are the most appealing symptom of all and, of course add a lot of drama to the story. But flashbacks are a symptoms of very severe PTSD. Some other common symptoms are anxiety, agitation, irritability, hostility, self-destructive behaviour or social isolation. All of these other symptoms are equally devastating. So throw in a little bt of these along with the flashbacks to add a touch of realism.
Show their inner turmoil.
People with PTSD try to hide their pain from the others. People stare. They avoid you. They whisper. The person suffering ca. act up any time in a situation he/she has been in a thousand times before. But before this happened, they try to soldier up for as long as possible.
Show the internal war going on inside you characters every time they experience these symptoms, trying to regain control, trying to defeat the thing making them weak. Internal dialogues help tremendously to show this.
The coping mechanisms.
People (and your character) try to always have a strategy to manage symptoms. These coping mechanisms help them survive these bouts of attacks. This mechanism could be anything, depending upon your character's preferences. They could have a mantra, a safe person, someone they trust to help in these situations. One way to show your characters growth is to show the removal of their dependence on these techniques.
Blindside your character.
With PTSD, you can never be sure. A situation that's been fine a thousand times before can trigger you one day because someone new is there, or you didn't get your regular seat, or something else of the same accord. This is a great device for a pivotal conflict.
Show your characters getting stronger, trusting people again, forgiving themselves etc. The whole process will be messy, two steps forward, one step back. Because let's face it, the stories that end with a pretty low bow and everyone leaves "cured" just doesn't sound authentic.