How would I argue that the Presidency is a lonely job and highly dependent on the advise of experts and the President is both Head of State and Head of Government, which is unlike parliamentary democracies, thus more than anybody can handle?
There are a number of ways in which we can argue that the presidency is not too big for any one person. Some of the most important are as follows:
Head of State duties are rather minor. The question contrasts the American president, who must act as head of state, with prime ministers who do not. But this is not a real distinction. Prime ministers typically represent their country in many of the ways that presidents do. The other occasions—things like ceremonial state dinners—are not serious burdens on a president’s time or emotions.
The president has a great deal of help. While it is true that the final decision on issues such as war do rest with the president, he (or someday she) has a great deal of help. There are plenty of advisers to give counsel. The president does not truly have to make decisions all by himself.
The president’s ability to control domestic policy is overstated. While people hold presidents responsible for domestic policy, this is not something the president can really control. The burden of actually making laws rests with Congress. The president does have the task of trying to persuade Congress to act in certain ways, but it is not as if the president is making decisions on a day-to-day basis that control the fate of the economy.
For these reasons, the presidency is not too much for any one person.