as obstinate as a mule 固执；死心眼

【例句】
He was as obstinate as a mule on that point.
他在那个问题上非常固执。

There is not the smallest possibility that, after having been as obstinate as a mule for two years, she suddenly became amenable to reason.
她已固执己见长达两年之久，绝不可能突然间变得可以理喻了。