Mathilda question

Dear community: I am desperate, on my trip to Gran Canaria I picked this book that I could not finish reading, I stayed half way, it was of red cover, very old, with many pages, maybe 700 or more. The title was the name of a girl, the protagonist, I was so sure it was "Matilde" but it is not. I remember it is written in first person, since she was in school (christian school, she was very egocentric and of hard character and used to explain she would get jealous of the Maria character being always mentioned as the greatest woman in the Bible so she used to pray changing Maria for her name, it was hilarious). Later due to the II World War, she separates from her family when traveling to another city. In her way she decides not to join her family so soon and in the way she meets some interesting men (soldier, a coronel) that introduced her into the war scene and at some point someone tried to make her a spy, here I could not continue reading it. It has been 6 years and I want so much to find this book! Please can anyone help me?