The fashion entrepreneur said Ivan's premature death in 2009, the year before David Cameron entered No 10, was of such magnitude that it made everything else "irrelevant" and "meaningless".
Ivan, their first child, was born with a severe form of epilepsy and cerebral palsy and died shortly before his seventh birthday.
In an emotional interview for The Times she described how they had to cope with regular "life or death situations" throughout the youngster's life, yet it came as a "huge shock".
Mrs Cameron said she sought strength from her faith: "Being a New Testament Christian I have found very grounding. It helped me to think I had been given Ivan as a gift to look after."
She added: "Ive dying is such a massive thing that everything else is irrelevant. It just overshadows everything. What goes on in the outside world becomes meaningless.
"Like anyone else in my situation, I just kept going. You have to deal with it, because you have no choice."
She said the couple relied on routine so they did not "fall apart completely" and they still visit Ivan's grave "very often".
