But I perceive in you so excellent a touch of modesty, that you will not extort from me what I am willing to keep in; therefore it charges me in manners the rather to express myself. This sentence says that because Sebastian can see that Antonio has the good manners to not demand to know what Sebastian doesn't want to tell, Sebastian's own good manners demand that he tell Antonio what he would rather keep to himself.
    The sentence represents a sudden reversal on Sebastian's part. From the beginning of the scene it's obvious that Antonio is concerned about Sebastian and would like to look after him, but Sebastian refuses his help. Antonio then asks him to at least say where he is going. Sebastian replies that he's just going to wander about, but then decides that he will "express" himself to Antonio. And so we get the information that he believes his twin sister is drowned and that he is headed to Count Orsino's court.