One of the core features of PTSD (post traumatic stress disorder) is hypervigilance, or being on guard all the time. What happens is that when you are triggered by any reminders of the trauma, then you may start to have nightmares or flashbacks about the trauma. This then induces anxiety, and you then try to avoid any future triggers, in an attempt to not feel so anxious. However, the avoidance maintains the belief that the trauma is still occurring, and hence you continue to scan your environment for any signs of danger.
So the way to reduce your hypervigilance is to break this negative, vicious cycle of PTSD, by thinking more adaptive thoughts, and not engaging in avoidant behaviors.You can also talk to your doctor about medication treatment, especially if you do not respond to psychotherapy, or if your symptoms are severe.
The following video explains more about PTSD and hypervigilance, and what you can do to reduce it: