For perhaps the psalmist is telling us that we need to go to this place of pain. If this kind of fear does not cause us pain, then we aren't really allowing ourselves to experience it. We are still running from it.
If we acknowledge the reality of what has seized us. If we stop in our tracks and experience it, we can then truly feel the fear as well as the pain caused by the sense of separation from any sense of oneness and connection with God and the universe. We are utterly alone. No one is there with us. Our ego has done it's job and convinced us that we can do it alone. But we cannot. No one really can. And it is this realization that - if we allow ourselves to truly experience it - can metaphorically or actually throw us to the ground writhing in pain.