Obtain an amethyst, large enough to write upon.
On it draw the rune "Ehwasz" (which resembles the letter"M"). Holding it first upside down (so it resembles a "W", with intent that it will reduce and block your nightmares, rotate it right-side up (now looking like an "M") and upon it and down through the middle draw the bind-rune "Isa" (a single vertical line). Placed under your pillow, this should hold off nightmares (add in a relatively non-spicy diet) and that should do it in the short term. In the long term, this will just be a hold/block. You must work your own way back to the root cause of your nightmares, through, perhaps, divination aids such as the Tarot or I-Ching, or counseling of some kind. This is, at best, a band-aid to give you some peace while you take on your inner journey. You might also wish to give Lady Mimi a call at her store. She can be quite helpful.
Best wishes --