The comedienne, 53 ? now in remission ? told how drugs, chemotherapy and radiotherapy left her "drained and exhausted".
She said: "It's like suddenly becoming older.
"You feel fagged out, you lose your motor and it makes you feel depressed.
"You have that 'I want to go to bed and sleep for ever' kind of feeling."
The French and Saunders funnygirl ? interviewed in the Radio Times ? said that just as she was at her lowest ebb she was also hit by the menopause.
She described it as "like jumping off a cliff" ? and she felt "the whole world" was against her.
The mum of three, who is married to comic Ade Edmondson, was persuaded to seek help for her mental problems.
She described the medication she was prescribed as "brilliant".
Jennifer was also buoyed by teaming up again with Absolutely Fabulous co-star Joanna Lumley, 65, for three Christmas episodes of the smash BBC sitcom.
She now ensures her daughters Ella, 25, Beattie, 24, and Freya, 21, get yearly mammograms ? telling them: "Just do it."