The Presidency is a lonely job and highly dependent on the advice of experts. Unlike parliamentary democracies, the President is both head of state and head of government. Is this more than anybody can handle? Is anybody ever really prepared for this awesome responsibility?
The presidency may well be a job that is too awesome for anyone to handle. It may be true that no one is ever ready to be president. However, if this is true, it is not because of the fact that the president has to act as the head of state as well as the head of government. There are at least two reasons for this.
First, there are many ways in which the Prime Minister of a country like the United Kingdom has to act like the head of state even though he or she is not. For example, Prime Minister Cameron of the UK visited the site of the Boston Marathon bombing recently to lay a wreath there. He also made a statement after the killing of Lee Rigby, the British soldier murdered by Muslim extremists, that was really just a statement meant to encourage the country. It was not a statement of policy from a head of government but a statement of resolve by a head of state.
Second, it is not as if the duties of a head of state are terribly onerous. The job of the head of government is extremely difficult, but the job of the head of state is not. The head of state typically does ceremonial things that are meant to create solidarity in the country. These are things like visiting the victims of tornadoes. These are not things that take a lot of effort.
For these reasons, it is clear that the presidency is not that much harder than being prime minister in a parliamentary democracy. It is also clear that the tasks that go along with being head of state add that much to the job of head of government.