In looking back at his own life, Wordsworth notes that Nature never did betray [his] heart.
From Sarah Duncan: Tintern Abbey
A great deal of the poem details the maturation of the poet in body and imagination, setting up a familial parent/child relationship between Nature and Wordsworth. The poet sets out four main stages of development: childhood, adolescence, manhood, and that which is yet to come, presumably old age or death. / ... /
... the final stage of his maturation, looming in the dark, which is manifest as the poet’s fear of being forgotten and of forgetting this scene of refuge (ll. 58-61). He begs Dorothy not to forget this scene, as he can see his own youth in her eyes (ll. 116-119, 148-9), and to know that it was dear to him because of her (l. 159). Ironically, as Bloom notes, Dorothy is only a year younger than the poet, who is himself not out of his twenties at the time of this writing. "His imagination aged very quickly, and Dorothy’s remained young and perpetually receptive to the beauty of the natural world" (411). Perhaps this quick maturation worries Wordsworth, and may have led him to think that he was on a quicker road to the end than actually was so.
Yet, he is grateful to his parent, nature, for guiding him through these stages, and encountering him wherever he stood. Even as he grows older, he is "well pleased to recognise (sic)/In nature and the language of the sense,/The anchor of my purest thoughts, the nurse,/The guide…." He thus entrusts his sister to this parental facet of nature, saying, "Knowing that Nature never did betray/The heart that loved her; ‘tis her privilege,/Through all the years of this our life, to lead/From joy to joy." He then goes on to encourage Dorothy to stay wild and allow nature to fill her with "quietness and beauty" so that she will never be harmed by the ugliness of life.