I'm all at sea.

我是一片茫然。

I'm throwing in the towel.

我认输了。

My heart is heavy./I'm heavy-hearted.

我心情很沉重。　

I'm not too happy about it.

我对此并不乐观。

It's hopeless.

没希望了。

It's the end of the world for me.

对我来说这简直是世界末日。

No such luck!

没那么好运气！

I'm downhearted.

我感到灰心丧气。

I don't feel like doing anything.

我提不起精神做任何事。　

I'm on my last legs.

我真是山穷水尽了。