In this poem, love inspires the speaker to try on new identities in order to try to find one fitting for his new role. The new shirts he buys are the symbols of this: one is green, one is pink, one is "tan / with darker stripes." When he puts them on, he hopes to catch a glimpse of himself reflected in a store window. He hopes the images will allow him to see who he is supposed to be. He keeps trying to "surprise" his beloved with "changes." The love affair does not work out: his beloved moves on, but he still thinks of her.
While he does not say this explicitly, we intuit that the speaker loses his beloved because he didn't know who he was. He suggests this in the first lines of the poem, when he says,
That year I had nowhere to go, I fell in love—a mistake
The "nowhere to go" implies that he was lost, rootless, at loose ends when he met her. Being in love sent him on the path of self-reflection, but it didn't get far enough. He tried on new outward identities like new shirts and tried to let her choose what she wanted him to be. That strategy, however, didn't work. The implication is that you need to have a deep inward sense of who you are before you can build a successful relationship with someone else. It's more than just trying on new "shirts" or external selves. The speaker, however, continues to self-reflect and has learned enough to let his beloved go, although he still loves her.