For the thing which I greatly feared is come upon me; literally, for I fear a fear, and it comes upon me. The meaning is not that the affliction which has come upon him is a thing which Job had feared when he was prosperous; but that now that he is in adversity, he is beset with fears, and that all his presentiments of evil are almost immediately accomplished.
The second clause, And that which I was (rather, am) afraid of is come unto me, merely repeats and emphasizes the first (see the comment on verse 11).