I feel really down/low about it all.

我对此感到非常失望。

He's in very low spirits.

他情绪十分低落。

He's in the depths of despair.

他失望透顶。

My heart sank when I saw him.

我看到他时心情一下子变得沉重。

They looked very down in the mouth/down in the dumps.

他们看上去闷闷不乐。