As hard and cruel as it might be - just finish it by any means necessary. I've been in the same boat, when I was producing trance as my main musical output I had a piece I was working on that was by far the best track I've ever produced. I got about four minutes in and I had no idea what was going to happen next. I listened back and listened back for ages, tried a few things that never seemed to fit.
This carried on for over a year. The amount of times I fired up Ableton and listened to it once, then powered everything down was heartbreaking. From time to time I'd start a new track, but I'd only get as far as finding a great kick sample, and on the odd occasion I made a bass patch. But my heart wasn't in it, all I could think about was this other track. I hit a real rut, even contemplated selling my gear and giving up.
I met with a producer who's inspired me quite a bit over the years at a party we were both DJing at, and I was telling him my problem. He told me that if I didn't finish that track, that I may as well quit music altogether. He told me to finish that piece, and then make another finished track the same day. Make four tracks a day if I had to, just get them finished.
At first I was skeptical, I wanted that track to be the dogs bollocks - not some rushed piece of music that I finished and forgot about. However, if it was finished, it was finished, and not just sat on my hard drive ruining my passion for music...and he was right. It isn't the finished product I wanted, but it was complete. Then I finished another track, and another. And the frustration was gone.
I don't find that I finish everything I start, like I tried to back then, but I know now that it can be done. I feel that getting over that block was a massive change in my life. I feel excited to work on music again, and I know that if I need to, I can get it done.
So tl;dr - finish it even if it sucks, you'll feel better about it.