[link to original on tumblr]
---&---
In both verses, there's a sort of parallelism. In the first verse, "When I'm thinking back, I'll remember" is balanced with "When I'm thinking back, I won't forget you." So there's "I'll remember" and "I won't forget," which say the same thing but in sort of opposite directions.
In the second verse, there's "When I'm thinking back, I'll remember" and "To when I last saw you; I can't forget you." The beginnings of the lines aren't the same, but the ends exhibit the same thing as those in the first verse - saying the same thing but in different ways.