Isn't the definition of the Threefold Law basically Cursing yourself if you curse?
Don't take this like I'm saying that you Cursed anyone. I'm not! I just want to make sure that you know that, in times of distress or misfortune, you may end up wishing someone was dead, or just "Gone." Or just some other negative mindset. With the knowledge we have, we know that spacific things are controlled by our outlook on them. Maybe that knowledge that we're supposed to have makes us more liable for our thoughts and actions.
Becouse, what is Magick besides requesting something to happen by concentrating our Body, Mind, and Spirit into the spell's action? If we, perhaps for one second, wished something bad enough to happen then it just might. So, now, consider if you were wishing something were "Gone" or some other way like that. Then that thought might be construed and the Universe would think that it's valued customer wants something to happen... Likewise if it happens or not, you'd probably retain that negative energy.
In a sense, Negative thinking is the most broadest and the most loosely defined as Black Magick, becouse it acts like the stuff. It's not as bad as the real deal, but it can lead to bad things happening. And, like Black Magick, only good things can keep you from being sucked into the all-accursed loop of Black Magick (You cast a curse, retain the negative energies. The Negative energies lead you to getting mad and casting Black Magick again, and therefore retaining even MORE Negative Energy).
As I said, Negative Thoughts are lighter, but the idea of Negative Thoughts leading to Negative Things which, in turn, lead to Negative Thoughts is just the same loop.
Basically, I would do what everyone else said. I, Myself, would just take a day off and lay back with my girlfriend, maybe *unn-tss-unn-tss* then sleep. The Thoughts themselves would have been gone away for so long and be so far from my thoughs that they would loose their hold on me. The main cause for this kind of bad energy is almost always Stress. Which leads to the bad thoughts.