My shame and guilt confounds me. Forgive me, Valentine: if hearty sorrow Be a sufficient ransom for offence, I tender 't here; I do as truly suffer As e'er I did commit. (5.4.8)
After he's caught trying to rape Silvia, Proteus apologizes to Valentine, but not for his crime against Julia. Here, he apologizes for being a lousy friend to his guy pal, as if trying to rape Valentine's girlfriend was a crime against Valentine, and not Silvia.