I hear you on a lot of this! (Yesterday evening was spent poking feebly at the last two scenes of this 15K+ fic that I have been trying to finish for something going on five years now, but those last two scenes simply won't gel to my satisfaction.) I'm always grateful when the plot ideas I have are the ones where the ending is one of the first things I think of, and then the bulk of the work is to figure out how to get the characters to that point.

And ahahaha, oh, do I understand the periods where writing scarcely happens at all. For me, they usually come when work or something similar has completely drained my creativity. I'm definitely one of those people who looks on my own personal creativity as something like a well or a pool -- if I don't draw too heavily from it, it'll keep refreshing itself, but if I'm forced to pull from it too much (e.g., I have a lot of very technical work-editing to do that demands my complete focus and concentration) it'll run dry. I'm just barely edging out of a long dry spell now.

I hope you're able to finish something to your satisfaction soon. ^_^