I am an optimist but there is a saying in the nuclear fusion industry. "Fusion is 20 years in the future and it always will be.

Fusion has been elusive because nature has a way of protecting the universe from too much instability. Nuclear reactions release a huge amount of energy compared to anything that we see on earth. Fortunately they are slow.

The reason for the slowness on earth is the same reason that fusion happens slowly on the sun; electrostatic repulsion. Bringing fusible nuclei close together means overcoming the repulsion between positively charged protons. The force is huge and on earth as on the sun, it essentially cannot be done.

It cannot be done but there is fusion at very, very low rates. Fortunate that nature protects by the near impossibility of fusion or we would have long been incinerated by the sun. Fusion happens by quantum tunneling. The is a behavior that allows for a VERY small percentage of fusible nuclei to actually jump across the electrostatic (coulomb) barrier and fuse with another nucleus.

This is a pathetically low probability of happening but if you crash enough energetic nuclei at each other on the earth or on the sun, then you get some fusion. For this reason, fusion is a VERY difficult proposition. Even in the brief flash of a hydrogen bomb, only a tiny fraction of the nuclei fuse. It is terribly inefficient.

The scientists are working hard but it is nearly like trying to invent a perpetual motion machine. Not quite as impossible as that but extremely challenging for sure.