Nightmares can sometimes happen when you unconsciously think of what happened to you. Maybe you haven't gotten over the problem yet, maybe you haven't faced it... Meditation isn't just about being calm; it's like a different state of consciousness. You focus your attention on a mantra, or on your breathing, or whatever you can use as an anchor. It's hard at first, but once you get it, a lot of the stress you're feeling goes away. Some people focus on the problem itself, like, meditating on an issue to see what to do about it.

I'm not very good at it, but I've heard a lot about it doing wonders for people.