Monica knew Khan better than he knew himself. He was her man, her idiot, so she was aware of his many shortcomings and how blind he was to them.
An ordinary warrior couldn't complete the quest that afflicted Khan's life. Even the best soldiers in the universe would probably fail at that, so Monica knew Khan needed to become greater than he already was. He had to evolve beyond the very concept of power and couldn't do it while being shackled to a mere human being.
As painful as it was, Monica knew what Khan needed and what she had to do to let him achieve that. It killed her even to think about uttering those words, but her life was nothing compared to her love for Khan. Monica would happily die and suffer many times over for her incredible, dumb, and selfless man if that got him where he had to be.