A job can be so frustrating when you are the only one who knows how to perform a particular role..

Judging from my experience everyday at work, I have been through a lot of stress lately due to the fact that I'm occupying a role which I alone can handle..
They are other smart people who can handle it too but they have their own jobs too and have to report daily before the close of business, so right now I have been asking myself these questions...

What will happen if fall ill?
What about me annual leave?
Why am I the only one who knows this apart from my supervisor?

All these has been my thought, it makes me ponder and get tired of what I'm doing, even when I'm down I have to try and go to work because if I don't they will be lapses which I might find so difficult to detect when I'm back..

Now I have complained but the system refused to employ another because we they feel we will be over staffed, the latest development was to tell me to work from home whenever I'm down...

Please friends, what do you advise me to do?