OMG I had that exact same problem with Stitch Two Together (Rose/Kanaya knitting fic). I mean, I only had the one ending in mind, but I did know exactly what was going to happen and what I had to write next, and yet the closer I got to it the harder it was to make myself sit down and write the damn thing. Eventually I got to the point where I just made myself plow through it because I wanted that fucking thing done. And then I ended up basically drained/with post-fic blues, which was new for me. Hope you don't get that, by the way; it's quite irritating.

Paul Simon has more on the subject of things getting harder to do the closer you get to the end.