@Kasa_Sama im sorry for suddenly replying in 2020, but im just here to say i seriously undertsand how you feel . it's like the idea is there and you kept thinking bout it always and suddenly you lost interest. What im saying is, dont worry. Each writer has their own pace. And if you really got that idea stuck in your head, simply write it in a book. If you plan on making a book, you can make it as a reference.
Sorry for making it long, hehehe. Hope something i said helps.