Sara, I know the feeling. I slso know that when I've gotten into that kind of thing it is often because of a mindset that I let myself cultivate. Something that I call 'poisoning my own well.' I rarely do it now but I'm still capable of it.

Like the others I suggest first finding out if there is a physical cause for it that needs treatment. If medication is not called for, then just plain doing something for someone else can do wonders. Visiting a housebound neighbor, or calling a hospital or nursing home and seeing if they need someone to read a story to young'uns or writing a letter for an old'un can send you home with your heart soaring. (Sometimes a nursing home resident with dim eyes has literally not had a letter read to them that they received a week ago, because the staff is so busy. Other times, an old person wants to hear a letter read and reread- staff rarely has time for that.)

That, combined with fresh air and exertion, may do the trick. At the very least it will buy you some time to think things through.

Music - making it, hearing it, writing it - is my first antidote, of course. A close second- or parallel to it is writing something original. Take a subject dear to you or close at hand and find out where it leads.

Good luck.