That's a terrible feeling and one that I am familiar with.
I tend to be in my "dark times" from December through February.
Be sure to reach out to someone. My father is the one whom I miss the most. He's been dead for 8 years and I still grieve. Some days are better than most but I talk to him every day - if only to say "I miss you."
Take the "bad thoughts" and write them out. You don't have to share it, but I've always found release in purging them outside of my body/mind/spirit.