Helium fuses into carbon through a process called the triple-alpha process.
The triple-alpha process to fuse helium into carbon requires a core temperature of around 100 million degrees Celsius.
And during the triple-alpha process three helium nuclei which are also known as alpha particles fuse together to form one carbon nucleus.
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
In stars heavy enough, after the supply of hydrogen is exhausted in their cores, their cores (or a shell around the core) start fusing helium to carbon.
In the most massive stars (at least 8–11 solar masses), the process is continued until some of their energy is produced by fusing lighter elements to iron.
Fusing hydrogen to helium releases energy.
Splitting helium into hydrogen takes energy.
Fusion is more or less the only source of energy in the universe, so no it's not possible to split all helium back into hydrogen because there is no appropriate energy source.