。
She couldn't overcome her mental anguish.
Literal
She [topic-は] mental-strain [to-に] couldn't-prevail.
打ち勝つ (うちかつ) is a compound verb meaning 'to prevail over, to overcome' — combining 打つ ('to strike') with 勝つ ('to win'). The potential negative 打ち勝てなかった means 'was unable to overcome.' 心労 (しんろう, 'mental strain, anguish') combines 心 ('heart/mind') with 労 ('labor/toil'). The に marks the adversary she couldn't beat.