Is writing the most difficult yet most rewarding thing to do in the world?

It depends at what you define "rewarding" as. If you think of a feeling of self-fulfillment is rewarding, then you're probably right. If you consider fame and an appropriate amount of money to live off of within your lifetime as rewarding, then you're dead wrong. Even F. Scott Fitzgerald, one of the most influential and famous American authors , died as a failure before he was truly recognized.