The problem is that that would break the laws of thermodynamics. What you described would work under the same incorrect principle as a perpetual motion machine. You can't just produce energy from nowhere, so the kind of cycle you're describing could not work - it would take far more energy to fission helium apart into hydrogen than we could hope to get out of it. That's also why the heaviest stars in the universe produce iron, and nothing heavier unless they supernova - once you hit iron, it takes more energy to fuse atoms than you get out of it.
By the way, what you're talking about with hydrogen and oxygen to produce water is something else entirely - a fuel cell. It doesn't involve any fusion, as it is chemically combining hydrogen and oxygen into a molecule, not combining two atoms to form a new atom. And while you can produce energy by combining hydrogen and oxygen into water, when you try it in reverse you once again hit thermodynamics - it takes more energy to break apart the water molecules than you could get out of it.
