I don’t know if that’s the case with your stories, but often people lose motivation because they haven’t thought the story through to the end, prior to writing. So, you get this idea. You get very excited. And you start. And then, somewhere into the first few episodes, you realize, you have no scene to look forward to writing.
If this applies to you, I would recommend, taking the time to write an outline first. I often also just put down a note on scenes I want to be happening later. For example let’s say: There is going to be a major drama scene, way later in the story. But I’m looking forward to writing it very much. I just write it down in a note app. This way I’m motivated to get the story to this particular point. By the time I reach the scene, I of course have to edit it again. But it helps me.
Hope this helps a little.