Phrases such as “while I lived” and “the grace, the beauty, the poetry” indicate most about the narrator’s feelings  that Twain was disappointed by his new feelings about the river.
When he says “While I lived” He referrers to the time he had spent in the river, the adventures, the excitement. Â
The grace, the poetry, the beauty describes the way the narrator has always seen the river, as a magnificent scenario full of life and susprises. A place where he has lived his most amazing adventures