The speaker is someone who has trained themselves not to say "love." The epigraph, "Never say the L word," reads like a rule they've set for themselves, and the title, "Harbor," suggests where it leads: a sheltered place where nothing gets in, but nothing sails out either.
The two short lines set up a pair of trades. Choosing "safe over sorry" sounds sensible, but "lost over loved" is sadder, because the speaker would rather be lost than risk being loved. The third line, "these are the choices, I've had to make," sounds weary, as if they don't feel they had much freedom. The closing image says why. Catching rain in a cup is simple, while catching it on the tip of your tongue is nearly impossible. Love, the poem implies, is that kind of thing: delicate and hard to hold, and easy to miss. The mood is guarded and quietly regretful, with a sense of longing under the caution.