当我们对一个人彻底失望时，内心充满了无尽的失落和痛苦。但是，有些时候，我们难以表达这种情感。下面是一些精选的英语短句，可以帮助我们更好地表达自己的内心：

1. You have let me down so many times.

你让我失望了很多次。

2. I thought I knew you, but I was wrong.

我以为我了解你，但我错了。

3. I can't believe you would do something like this.

我无法相信你会做出这样的事情。

4. You have shattered my trust in you.

你已经摧毁了我对你的信任。

5. I don't think I can forgive you for this.

我不认为我可以原谅你的行为。

6. You have shown your true colors and they are not pretty.

你已经露出了真实的面目，而它们并不美丽。

7. I never thought you would stoop so low.

我从未想过你会如此卑劣。

8. You have proven that you are not the person I thought you were.

你已经证明了你不是我想象中的那个人。

9. I feel like I don't even know you anymore.

我感觉我已经不认识你了。

10. You have disappointed me beyond words.

你已经让我失望到无以言表的地步。

这些句子表达了对一个人彻底失望的情感，帮助我们更好地表达自己的内心。无论我们选择哪种方式，重要的是要坦诚地表达自己的感受，并尝试解决问题。