Everything in this short poem is an apology, but the odd thing is what the speaker apologizes for. Nobody says sorry for loving someone, making them happy, or saying "I love you." These are things people usually celebrate. Here they're treated like wrongs, which tells us something has gone badly. The poem never says what happened, but the speaker seems to be looking back on a relationship that has ended or turned painful, and now feels that even its best moments were a mistake.
The speaker's feelings seem to be guilt, hurt, and exhaustion all tangled together. The repeated "I'm sorry" works like someone who can't stop saying it, as if no number of apologies will be enough. The line about "the tears I cried" is the quiet turning point: the speaker was hurt too, yet still ends up apologizing. The mood is bare and sad, and the poem's plainness makes it feel painfully honest. It begins and ends with the same two words, so the speaker has nowhere left to go.