How to calculate the gravitational force of an object beneath earth's surface?

According to Newton's Law of Gravitation, F=GM1M2/r^2, we can calculate the gravitation of an object beyond Earth or on Earth's surface, using mass of Earth, mass of object, radius of Earth and altitude of object.
When the equation is applied to object inside Earth (beneath Earth's surface) I got gravitational force of object at the centre of the Earth = infinity. But in fact, an object at that point only experience high pressure, but no acceleration, that is no net force. Where did I get wrong?

My physics teacher said that when object is beneath Earth's surface, the mass above that altitude doesn't count towards gravitational force. That means
M1= mass of object,
M2= mass of Earth x (radius of Earth - depth of object)^3/(radius of Earth)^3,
r= radius of Earth - depth of object

But why doesn't the rest of the mass count? Can anyone explain to me?