I can relate to this. I have always wished I could find one thing to be passionate about and become an expert on. I seem to become obsessive about one thing for a short while and then gradually loose interest. There seems to be a "honeymoon" effect for me. This goes for hobbies, side projects, work projects.

I think I enjoy the discovery and research aspects and have romanticized visions of the end goal but end up never going deep enough before moving onto something else.

I studied and worked in journalism for a while. This was interesting work to me, because I could dive into a topic for a story and then move onto the next one. It helped to be a generalist. Perhaps you can blog about being a generalist and whatever interest you at any given time. Maybe something will really resonate that will lead to deeper dives.

Having "seasonal" interests or ensuring you can leave and return to interests has helped me. Living in the PNW, we have actual weather seasons. I love to ski in the winter, grow a garden in the spring, be on the water in the summer and go to football games in the fall. I get excited as these seasons change and can easily leave the last behind, knowing it will return.

Similarly, when I get into something, I try to make it manageable to dive back into it without starting from scratch by maintaining things or information I have collected along the way. The foundations will be there.