Don’t stress out about it!!
The great thing about writing is that an idea that you love but lose interest in usually just needs to sit in the back of your head for a while while you develop it to something you obsess over! Or, as another suggested, you might combine it with another idea. You might have an idea about a high school romance and then another idea about a family drama mystery and end up combining aspect of both of those to create a story you absolutely love.
Just keep writing. Don’t be afraid of writing a ton and then deleting it - everything you write will develop an idea, one way or another. You can start a hundred projects and that’s completely fine. You’ll eventually find one that deserves your attention the most!