The short answer
You're not losing energy. You're discovering the project was built on excitement, not a real reason. Excitement burns fast. A clear, honest reason burns slow. The fix isn't more motivation — it's finding out why this project actually matters to you before you start moving.
You keep starting things full of fire, and then weeks in, the fire goes out. That pattern isn't a character flaw. Research on personality and self-regulation suggests that people who lead with enthusiasm often aren't separating the thrill of beginning from a genuine commitment to the work itself. The launch feels like progress. It isn't. The early energy is real — but it belongs to novelty, not to the project.
Marcus put it plainly: strip the thing down to its bare cause. Not what it looks like, not how it feels when it's shiny and new — but the actual reason it should exist in your life. When you look at a new project, you're looking at its surface. The burst of energy is a reaction to that surface. The moment the surface becomes familiar, the energy has nothing left to feed on. What you need underneath is a cause that still means something when the excitement is completely gone.
This week, pick one active project — something you haven't abandoned yet. Write down, in one or two plain sentences, why it still matters if it never gets noticed, never gets praised, and takes twice as long as you thought. If you can write that and mean it, you have a real reason. If you can't, that's useful information too — and it's better to know now than three weeks from now.