I don't even have time to catch my breath.
我连喘口气的工夫都没有。

catch my breath 是喘息，歇口气

Don't have too many irons in the fire.
不要揽事太多。

have (many, too many, several, other) irons in/on the fire意思是“同时有（许多，太多，好几件，其他）事情要做。

I'm in the middle of something. Could you call back later?
我现在很忙，你能过会给我打电话吗？

in the middle of doing sth. 正忙于做某事

He is working so hard that he is as busy as a beaver.
他没日没夜地工作，看起来很忙。

Beaver除了海狸的意思外，还引申为卖力工作的人，兢兢业业的人。

It's hard to slow down when you are trying to do a million things.
当你想要干成百上千件事情的时候是很难慢下来的。

a million往往代表“很多，超多……”。