I was under the wrong impression
間違っていた
I was mistaken
I had a misunderstanding
I had a misunderstanding
勘違いしてたわ。
大間違いをした
was completely at fault
誤解をとく
I misunderstood that.
言い損いだ
It was a slip of the tongue―a lapse of the tongue.
I was mistaken about that.
I was mistaken about that.