How do i stop myself from having nightmares?

Nightmares: Depending on how severe or frequent this is, you may need help with your nightmares. They could be a symptom of ptsd or other personal problem that needs attention. A psychologist or other therapist could be a good resource in assessing what's happening with you, and in working through it. There are also some medications that might help -- a psychiatrist or sleep medicine doctor can advise.