Usage

It is not standard formal English, but it is the kind of colloquial phrase you might hear in everyday speech.

Thank you. How should I say it properly then?

In an informal conversation I've got my thinking completely mixed up would be readily understood. The best phrase would depend on the situation, if you needed something less colloquial, but you could say something like:
I am confused.
I misunderstood.
I am mistaken.
My idea was incorrect.

Blue Jay It is not standard formal English, but it is the kind of colloquial phrase you might hear in everyday speech.

But is is standard spoken English.