The more you delay telling her, the more she will suffer.
The later she will find out about this, the more she will suffer.
なればなるほど
the more. . .、the more. . .;
(something increases or decreases in proportion with something else)
if you increase .... , you also increase
“the more+文書, the more+文書”という形を使うことで、「○○が□□をすればするほど、△△も◇◇する」という意味になります。