"Tintern Abbey" is a both a classic example of Romanticism and a classic example of Wordsworth's artistic aesthetic. It contains extensive reflections on not only the beauty of nature, but also its sustaining power, as Wordsworth meditates on the ways in which memories of his youthful adventures in nature have kept him company during his adult wanderings.
The "loss" that Wordsworth refers to is therefore the loss of his youthful innocence, especially as it is reflected in his boyhood adventures. However, while there is certainly a sense of bittersweetness present in this realization (Wordsworth is, after all, sitting all by himself in the middle of nowhere in a rather forlorn fashion), the poet actually views this loss as a relatively positive process. Indeed, Wordsworth says "for such loss... abundant recompense." It becomes clear that this "abundant recompense" is his adult intellect, his poetic power, and his more mature outlook on things. As such, though Wordsworth certainly looks back on his past with nostalgia, he still seems to look forward to his future with anticipation.