I hate to be a party pooper, but I am really tired.

我真不想扫大家的兴，不过我真的太累了。

I am not going to be the only party pooper here.

我才不要作为这里唯一的扫把星。

There is a similar phrase — party pooper.

Party pooper是在派对上扫大家兴的人！

Party pooper, but I have to go home now.

真抱歉要扫大家的兴了，可我现在就得回家了。

A party pooper is someone who (always) ruins the fun.

这个习语指的是总是在派对上扫人兴的人。