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The problem is that fusion is fundamentally a process that occurs within stars, and those things aren't small. They must reach a certain minimum mass in order to start the nuclear fusion process, otherwise they'd just be like planets. Gravitation is a necessary ingredient to the recipe to make it work, and stars are basically like huge, expendable fusion energy batteries that last for a couple billions of years, and exert extreme amounts of energy into all directions. Their death cycle occurs by an imbalance of gravitation and fusion that leads to a supernova and a transformation of the star into another type.

Doing fusion without that gravitation makes it impossible to make it sustainable. The gravitation itself creates a pressure large enough to run (force) nuclear fusion, and we have to (if it is actually possible) do it ourselves with "Earthly" means where we can't even utilize our gravity for it. That means chemical-physical processes.

It's easy to start a fire in the summer at day, but hard in the winter while snowing in Antarctica at night. I am not even sure if this metaphor comes close to the discrepancy with fusion, it might be worse by a further factor.

It is possible that fusion energy will NEVER be possible on Earth - at least not reasonably and sustainable. Just changing the parts is extremely expensive, which will degrade over time. A fucking treadmill is more cost-efficient than that.