We all know the situation: we have a problem that is so enormous it eclipses everything else. People with a serious illness spend a period when they can think of nothing else. People who once had lots of money and now have none, spend a period where they think if they only had money all other problems would somehow evaporate.
If it is something that can somehow be solved, the good thing about idée fixe is it allows focus and concentration on that problem. But what about when the idée fixe can't be solved?