Katja Schneidt initially embraced Mrs Merkel's famous rallying call "we can do this" after the Chancellor opened Germany's doors to Syrian refugees last summer.
But the 45-year-old has now written a book – titled We Can't Do This – expressing her anger at the country's dire integration policy.
She wrote: "The euphoria at the beginning of the wave of refugees is gone. We are not able to cope with this refugee crisis successfully."
In her voluntary role, Ms Schneidt – from the state of Hesse – helped refugees find accommodation and receive medical treatment.
But she now believes it is impossible to integrate every refugee into the country, which has been rocked by Islamist terror attacks this summer.
She said: "You cannot expect the refugees to behave in a way that we want them to after an integration course. That is nonsense.
"Islam is not like Christianity. It is a life model, we can't argue them away. It is not enough to say that Islam belongs to Germany."
According to official figures, nearly 1.1million refugees arrived in Germany in 2015, with a further 300,000 expected this year. [Daily Express] Read more
