Why do our first ideas seem exciting at first but turn out bad once we start working on them?

Hi there, has anyone else had the same issue? When I first build my project, it's like juggling balls. I come up with a great idea and think it can generate a lot of money and that people will use it. I jump into the code without designing, researching the market, and so on. I think I can iterate later, but in the middle of the work, I realize it's a bad idea. Then, I find thousands of similar software out there. In the end, my software feels like garbage and I don't finish it. Yeah, that's my story. I found that doing research and sharing the idea with friends can help a lot. How about you? Share your story here.