Disappoint
使人失望

I hated to disappoint her.
我不喜欢让她失望。

He disappointed me.
他让我失望了。

Let someone down
使某人失望

He let me down.
他让我失望了。

I let everybody down.
我让每个人都失望了。

I know it's silly, but I feel like everyone lets me down when I really need help.
我知道这很傻，但是我觉得当我需要帮助的时候每个人都让我失望了。

Fail someone
使某人失望

I'm sorry. I failed you.
很对不起，我让你失望了。

I failed the company by losing our biggest client.
我失去了公司最大的客户，我让公司失望了。

You failed her by not being there when she needed you the most.
你在她最需要你的时候不在她身边，这让她对你失望了。

Not live up to someone's expectations
没有某人期待的那么好

I didn't live up to my dad's/parents expectations.
我没有达到我爸爸/父母的期望。

Fall short of something
没有达到需要/期待

One or two songs on the album are interesting, but most fall short of the mark.
这张专辑里有一两首歌还挺有意思的，但是大多数歌不够好。

He would sack any of his staff who fell short of his high standards.
他会开除任何没有达到他的高标准的员工。

Fall somewhat short of someone's ideals.
达不到某人的理想

In appearance, she fell somewhat short of his ideals.
看外表的话，她有些达不到他理想女友的标准。