weak-willed
adjective uk/ˌwiːkˈwɪld/ us/ˌwiːkˈwɪld/
意志薄弱的；优柔寡断的
not having the determination that is needed to continue with a difficult course of action
My diets are never successful - I'm just too weak-willed. 我的减肥计划从没有成功过——我就是意志太薄弱。