How to sleep after seeing a death? Falling to sleep is a struggle I don't have nightmares, just cannot naturally fall to sleep

You might consider. Therapy to help work through this. Use good sleep hygiene. Retire ; rise same time each day. Keep bedroom dark ; cool ; use only for sex ; sleep. Turn off tv. No naps. Exercise regularly but not in late evening. No caffeine for 6 hrs ; no alcohol or tobacco w/i 2 hrs of sleep. Keep dinner moderate sized ; finish at least 2 to 3 hrs before sleep. Can try warm milk, chamomile tea or melatonin.
Grief counseling. Sorry to hear of your loss. You seem to be linking the death you saw, with your difficulty falling asleep. What is the connection for you? Some people fear they will die if they fall sleep. Is this like you? If so, death and sleep are very different states, and one does not lead to the other. Grief counseling may be helpful to you, as well as talking with trusted others about what you saw.