I had this problem all the time (at one point I was considering "never finishes anything" my trademark) until I finally finished something (this year). I'd left it abandonned for about 3 years but I still thought about it a lot, and realised I was making myself depressed by just leaving it there, since it seemed like something I should've finished ages ago, and felt like a game that described me really well. If I didn't finish it, all the work I'd done and all the words I'd written would go to waste and essentially it'd just be a series in-jokes that only I knew.
So I realised I was procrastinating, and I'd just come out of an unsuccessful college course and had lost all direction and was unemployed. I was spending days just staring into space. I decided to pick it up, not think about how much was left to do, and just work on it until I lost interest again. It took a lot of pushing myself, but once I was in full swing, it became a lot easier. (I also took way too few breaks for fear of losing momentum.) Two years later it was finished.
I think it was a combination of several things- my frustration at myself, and the fact I had nothing else going for me. I'd wake up and think "why isn't this game finished yet?" every morning, and worked on it because I couldn't focus on anything else.
...I wouldn't recommend this exact method to you. It was really stressful, and I barely socialised. I didn't take many breaks. I also exacerbated my OCD a lot. It's better to balance socialising, working on the game, and playing other games/relaxing. Towards the end of development I managed to do this, and was also working on a few side projects in case I felt like a change of scenery. The side projects AND the main project both benefitted from balancing what I worked on.