The speaker looks back on a past relationship and, instead of mourning it, thanks it. Calling the relationship a "war" where "none of us ever lost" suggests it was intense and sometimes painful, but never one-sided. The partner taught the speaker how to love and how it feels to be loved, and together they built emotional "walls" around the speaker's heart. The poem implies the relationship has since ended, though it never says so outright. The line "in case you ever leave" hints at it, and the speaker now knows how to protect themselves.
The feeling here is mostly gratitude, with a thread of sadness running underneath. The speaker admits to lost sleep and tears, yet wouldn't undo any of it, because it shaped who they are. The final lines land on the poem's main idea: the year and a half wasn't wasted, since it taught the speaker to love themselves. The mood is calm, honest, and quietly hopeful.