Fusion is already being done. The only issue is nobody has figured out how to make it produce energy.

The plasma is insanely hot, so it has to be trapped magnetically in order to not melt everything. However, magnets do not just hold the charged particles in place, it causes them to move. They use a combination of magnets and injecting neutrons and other techniques to try and hold the plasma for as long as possible. In addition, the entire apparatus takes a beating and I believe needs to be repaired every time they run it.

ETA is still unknown.