Don't seek forgiveness. That makes it seems as though you're only trying to alleviate your own guilt instead of being concerned about your cousin's welfare. The guilt is your burden to bear for such a horrible abuse of trust. If he wants to forgive you, he should be allowed to do so on his own terms instead of you trying to force it out of him.
I only hope your cousin finds strong support elsewhere and you take a good hard look at how you treat others from now on.