Steve pales instantly. It feels like the other man read his diary or something which is stupid. He stopped writing down his thoughts when he properly took the mantle of Cap. He can't know that very thought drove him to sit a block away from Stark Tower once upon a time before he defaulted to cynicism and left.
He opens his mouth and shuts it a few times before confessing softly, "I know." The look on his face and tone make it clear he didn't think Tony did. Rather than play it subtle, he follows Tony's lead and continues. "I didn't think you saw it that way so I never said anything. It felt... I felt desperate so I just...." Said nothing. Sensing a theme here?