I think what you're describing is something many of us who have Bipolar and depression face. There are times I'll sit down to do a practice piece, because my main project stories are just not talking to me, and they turn into some dark story, that I wonder where they came from. Those get shifted quickly to the "Never to be published" file, and I try to find something else to write about, or I put some music one or something to try to change my mood. I have spoken with my therapist before about this, and he's always wanted to talk to me to find out what the cause of them was.
I hope you can work it out for yourself. I don't have any words of wisdom to give you, but I will wish you all the best.