I have that problem all the time. I'll literally be sitting in class or on a bus and I'll have an idea for a story. I'll think it's fantastic , go home, write up a few chapters and then be like... meh.
My advice would be to literally write out everything that's going to happen in your story on a peice of paper. If you can throughly finish your story with bullet points or whatever you want to use, then go ahead write it. Because at least you'll know that even if you lose interest, you've still got that peice of paper that you can go back to and use to finish up the story even if you don't like it any more.
Also, I would actually write up as much as you can in Word. If you've gotten at least four chapters down without a dip in your excitement for the story, then you've probably got a good idea going.
My advice would be to literally write out everything that's going to happen in your story on a peice of paper. If you can throughly finish your story with bullet points or whatever you want to use, then go ahead write it. Because at least you'll know that even if you lose interest, you've still got that peice of paper that you can go back to and use to finish up the story even if you don't like it any more.
Also, I would actually write up as much as you can in Word. If you've gotten at least four chapters down without a dip in your excitement for the story, then you've probably got a good idea going.