If you are mistaken about something, you are wrong about it.
I see I was mistaken about you.
我明白我错怪了你。
A mistaken belief or opinion is incorrect.
I had a mistaken view of what was happening.
我对于所发生事情的看法是错误的。
if I'm not mistaken
You use expressions such as if I'm not mistaken and unless I'm very much mistaken as a polite way of emphasizing the statement you are making, especially when you are confident that it is correct.
I think Alfred wanted to marry Jennifer, if I am not mistaken.
我想如果我没搞错的话,当时艾尔弗雷德想娶珍妮弗。