To be honest, Shepard was used to gravity. She was used to the weight of her actions. Still, she remarkably adjusted to her surroundings and slew down her foes. Despite being a woman of war--she was still human, and over time fighting started to tire her, although she tried not to show it. Her weakness, her limitations, was something that she wasn't proud of--and so she powered forward, deciding that she'd rather die than give in to them.