I'm really upset. /I feel bad.                                                

我真的很烦。

I'm sorry that my friend has to leave now.                          

现在我的朋友必须要走，我好不舍。

I'm really disappointed./It's really disappointing. 

我真的很失望。

I'm disappointed in you. / I feel bad because of you.

我真的对你很失望。