、。
To tell the truth, I think you're wrong.
Literal
True [possessive-の] thing [object-を] say [conditional-と], I [topic-は] you [subject-が] mistaken [quotative-と] think.
君が間違っている ('you are wrong/mistaken') uses ~ている for a current state of being wrong. と思う frames it as the speaker's opinion. 本当のことをいうと softens the directness of telling someone they're wrong.