She said the pain was so unbearable, 'all you really want to do is disappear yourself'.
The 45-year-old mum-of-three told how she has endured a 'tricky couple of months" and apologised for being 'a bit sleep-deprived' when she spoke at a Missing People charity fundraising gala.
Kate said: "What many people don't know is the impact that a child going missing has on you, your family and all those who know your child. You need to know that all that can be done is being done and you need to know that the world hasn't given up."
At the gala in London, Kate, who is an ambassador for Missing People, she said: "The highs are when you think there is news. And the lows... the dread of knowing what happened and the dread of not knowing what happened."
But despite her and husband Gerry's ordeal, Kate spoke of her delight that a billboard campaign she launched has helped find 100 missing people.
She said: "It's a truly amazing achievement."
Kate told how every five minutes a child goes missing in the UK.
She added: "Thankfully the majority are found within 24 hours but a significant number are not. And so for these parents the pain continues, the agonising days and nights without news."
Police have re-opened their inquiry into Madeleine's disappearance from her holiday apartment at Praia da Luz in May 2007, shortly before her fourth birthday.