It's a common progression for creative people, even experienced ones.
You can read up on a what a lot of other people have done to break through the point of losing excitement to do a track and actually do the work of finishing it, but I think in general what you really need to be told is just to FINISH YOUR DAMN TRACKS. No tricks, no shortcuts, just sit down and get ready to go through the bog of working on it regardless if you feel interested or excited by it or not. I say there's not much good a lot of more specific advice can do for you because it's all as unique a process to you as your music style is. You won't know what the best way for you to get "motivation" to get through it until you do it. And do it again. And again. And again.
To be serious about doing an art means to employ a dedication to its discipline - in other words, get ready to sacrifice lots of time that won't be fun. Everyone here that ever got successful at music had to go through the same ritual and just accept it as part of what we do. I get the feeling what you're running into is just losing that OMG THIS IS AWESOME feeling we all get when starting something and getting the idea this could really be something and then just quitting it when that "high" runs out. Without experience in getting through the other end of it and going through the meticulous stages of making sure every single little fucking thing in it is perfect, you won't get through that barrier.
The encouragement part is, once you start getting experience in doing it, it can really build up quick. In fact, I personally make it a point in the soundtracks and albums and stuff I do now is to start a bunch at a time, work on them until I feel like I've got them at a good place, and then go back and do the "finishing" part for all of them in a "stage". I don't get obsessed with making sure I focus and start and finish one track at a time anymore because I've personally found that I work better in "batch bursts", what I call them. I'll get a bunch of tracks up to "nearly finished" stages, then one day I'll just burst right through and do the finishing stages for all of them in a single day because where my mind and energy at is "finish the job". No more "come up with something that could be something" or "ok, start getting some next parts going for this shit", it's "ok, the composition and arrangement is done, now to finish them off".
So, in actuality, you accidentally painted yourself into a good corner where now you can work on building a shitload of "finishing" experience. Start with one track, take as much time to make it perfect as you need, finish it and release it. Then do the same for another. Then the same for another. Then do 3 with a made up weekly schedule for each. Release. Then another 3. Release. And so on.
I tell you straight, few things in the world are as satisfying as finishing a song after you went through the not-fun part of doing it. It's like a non-gross orgasm that lasts all day. Then you realize you're quickly getting better and faster at doing it because the satisfaction of finishing a song is also a major EXP. POINTS burst. It really feels like you level up the first time you get an Ocremix posted, then your first whole album finished, then your first album published by a small label and so on. And those experience points don't go away as quickly as some other achievements.
So there's your motivation - sit your ass back in that chair and get to work and you'll get day-long orgasms you don't necessarily have to wipe off, and then find out you secretly jumped from Level 1 to Level 8 in just a few days and secretly picked up some "abilities" along the way, like being able to hear missing notes, missing instruments, wrong instruments and whole wrong sections just by listening to your own WIP a few times.
You'll be surprised how accurate I am here.