The Mindfulness of Breathing meditations I learned when I attended a Buddhist temple helped me immensely with negative thoughts. It's exactly what it says on the tin: One is supposed to focus on the breath and clear all else from your mind. I never could do it and it bugged me, until I realized it wasn't about clearing your mind at all, but returning to center. When I did that meditation and my thoughts strayed, I "looked" at my stray thoughts, accepted that they were there and turned my mental self back to center and the breath.

Took a lot of practice to get to that point!