Getha had made up her mind. She had run the simulation over and over and then came to a conclusive decision that no matter how careful she is being, it doesn't make her opponent any less scary or stronger.
Her taking her sweet time to think up a plan is only going to mess with her head. She knows she views Sango as this undefeatable monster, and that is where the problem comes from.
Telling herself that her opponent is undefeatable is the first step to losing the duel against him before it even begins.
Getha herself has to believe that he is not undefeatable. She has to do away with the thought of how she views Sango, and instead carve an image of him from scratch, one without an opinion of what he is and what he is capable of.
She needs to make herself believe that Sango is just another sort of opponent, though strong, and needs to be put down.