Don't worry we have all been there. It is a very common situation. Firstly, do not try to complete a single project in one day. If you want to complete it in one day then use a reward system. There is a app called habitica it's really cool. It has a reward system that is really cool for these kinds of situations. You should try it.
Let me give you an example, If you are running in a marathon, the only way you can win is by taking small steps in a steady manner. Rushing will not get you anywhere. It may get you further than everyone but not for long. So similar to that you must have small milestones set for your app or anything.
Create a schedule for yourself, brainstorm ideas for your app on how it should work, write documents about the front end and the backend, it's functionality, it's UI, it's best worst use cases etc. Just get into building it. Slowly you will get an idea on how you must proceed with the app.
It is okay to think really big. I recently wrote a blog about a to-do list. I am currently just evolving it and adding features to it. I think about really impossible ideas, it drives me to pursue the impossible. That is what makes us better.
We all make mistakes when starting hobby projects, we must always start laying out the basics, like thinking about the app you are going to build. Writing documents because if we know what we want we would not get lost in the middle.
I hope this advice helps you. Really try out the Habitica app it is really cool.