How do I stop having these nightmares?

In October one of my best friends overdosed on heroin. He was in the hospital in a coma for a couple of months and died in early December. Ever since, I’ve been having these really vivid nightmares three or four or sometimes even five times a week, in which people I care about die suddenly and unexpectedly. The other nights I have nightmares about terrorist attacks (residual, I think, from having been a small child in NYC on 9/11). I sometimes take melatonin before bed which helps keep the dreams at bay somewhat, but melatonin no matter how small the dosage leaves me groggy in the mornings, and I hate having to rely on a pill.

I am also on Zoloft for anxiety and Vyvance for ADHD, but I’ve been taking them without problems for years so I feel like that’s unrelated.

Any help would be much appreciated!