The Times of London has an excellent article on the letdown often experienced by people when they've achieved a long-desired goal. Gordon Brown's current problems are used as an example. Quoting......
Brown viewed elevation to the Premiership as the be all and end all of his political career: it was the goal towards which he was striving from the moment he conceded the leadership of the Labour Party to his close friend. Anticlimax was inevitable.
Thatcher, on the other hand, was no less ruthless or determined, but her guiding ambition was to change Britain. Becoming Prime Minister was, in her schema, a stepping-stone to a grander destiny.
I think Brown had an additional handicap, in that since he had been part of the ruling government, he was (is) in the awkward position of having already implemented many of his ideas. The elder George Bush had a similar problem. "More of the Same" is not an inspiring theme, especially when the Same is looking tired.
The overall theme of the article is quite correct, however. Having goals is admirable, but when they are essentially closed-ended with no idea of where to go from there, attaining them can be disorienting.