They could have a future together. That's why she pushes him away so emphatically, so readily. They could have a future, a wonderful, beautiful future together. They could. But first, she would have to put aside her pride. She would have to let Mal in. She would have to let go of the things that have held her back for most of her life, and start fresh.
Everything would be different. Everything would be new and hard and she would have to make herself vulnerable.
She hates being vulnerable. She's not sure she could handle it.
So, she stays quiet. She pushes Mal away. Even if it means losing her chance at having a future with him. She has told him many times that he is stubborn and foolish, but in reality, that is her. That is her, and it will continue to be her, as long as they live. They could have a future together... if not for her pride.