My opinion on this is... If its good the first few plays, then its good (enough) & worth finishing. Seriously, FINISH THE TRACK. Just because you lost interest after the 500th play, doesn't mean the song isn't great. The thing as an upcoming producer that many of us seem to forget (Including myself, all the time) is that you're not going to make a "hit", by your standards every single time - AND THATS A GOOD THING. You always want to learn from each track you make, and progress, so at a certain point you're always going to want more.
The only thing I can suggest, is to try different styles of music you're creating. I was always a rock kind of guy, but when it came to producing I found myself more into making hip-hop tracks, as I can make them faster, and move on to the next, which keeps me from getting bored. I truly think any track, regardless of genre is going to get "old" after "X" amount of time working on it. Maybe work on getting tracks start to finish, faster.