Maybe you have been holding on to a grudge over something that you can't even really remember the reason.

【翻译】也许你对某件事一直耿耿于怀，而你却甚至无法真正记得怨恨的理由。

【分析】that you can't even really remember the reason 是定语从句，修饰a grudge，不是名词性从句，that用作关系副词，相当于for which。

这个用法是比较特殊，关系副词that有时相当于in which，on which，for which。尤其表示“时间、方式、理由”概念时。

I liked the way that she organized the meeting.  我喜欢她组织会议的方法。

(that = in which)

We never forget the day that Hongkong returned. 我们永远不会忘记香港回归祖国的那一天。

(that = on which)

The book has a certain allure that it is hard to find a reason. 这本书有一种难以解释的魅力。

(that = for which)