Quinn simply wanted to return to the days when she could have told Rachel that she loved her. She now knew that keeping it a secret would only cause them both years of heartache. So, she devised a plan. She would return to her high school years, dress up like the old her, kiss Rachel Berry and tell her that she loved her. Rachel would of course be confused and think that it was the Quinn from her time period and that would stir up the emotions that she already knew were there. They were already in love with each other. She just wanted to speed up the pace.
She knew that the past Quinn would be stubborn. But, she also knew that Quinn Fabray, no matter what time period, could not resist a persistent Rachel Berry throwing herself at her.