Wordsworth talks about his love of nature and how the experience and contemplation of nature can elevate the mind to "sublime" and "blessed moods" to the point of being like a "living soul" (sans the corporeal body). The spiritual growth: Wordsworth comments that his appreciation of nature and his ability to contemplate that beauty, inwardly, has matured.
While here I stand, not only with the sense/Of present pleasure, but with pleasing thoughts/That in this moment there is life and food/For future years. (62-5).
Wordsworth is no longer just sensually and passively enamored with the transcendental experience of nature and the immediate gratification of present experience. He is now wiser, understands it more and understands how memory is a place (an inner space) that can be a realm of nature in itself. The progression here is that he can be externally affected, as he was in the past, but also internally.
(The coarser pleasures of my boyish days,
And their glad animal movements all gone be,) 73-4.
In the lines that follow, Wordsworth notes that he no longer thinks of nature as a "thoughtless youth." He now considers the "still, sad music of humanity" and "all thinking things." So, his progress is two-fold: becoming more introspective regarding his personal affection and contemplation of nature and integrating this more mature introspection with considerations of humanity in general and the connection between a universal contemplation of nature. This segues nicely to the last section on his sister, Dorothy. He lives vicariously though the youth of her "wild eyes" but also hopes for her to get the same mature introspection of nature in her own old age; and for memories of nature, as well as nature itself, to comfort her in weary times.