I am sorry, I do not know what you want to express, because I always say cannot bear sth or doing. My English is not very good, but since you refer to couldn't, I think you want to emphasize Polite tone maybe, but in Chinese, there is no obvious difference, 不忍心is enough 不忍心去做.... the latter one, 做......实属不忍啊。we always use 啊,哪to express our pity.