The end of Nephi's discourse is filled with powerful compassion, and his deep spiritual pain for the hearts of his brothers.
Nephi's last sentence indicates that he is so full of the Spirit that he has become physically weak. This is a recurring theme in the scriptures, where the tremendous presence of the Spirit can have a debilitating effect on the physical body. In the conversions of Alma the Younger, and King Lamoni later in the Book of Mormon we have the spirit so thoroughly overpowering the body that they body appears dead (though the death/resurrection, repentance/forgiveness images are also quite important in those cases). Joseph Smith had first hand knowledge of the draining effect of overwhelming spiritual manifestations.