Written late at night from bed, "Im sorry" is a confession the speaker can't bring themselves to say out loud. They love someone who doesn't love them back, and the whole poem is an apology for that hope, for whatever was said or done, and for feeling so much. The speaker admits the other person has made things "fairly clear," and then takes all the blame: "There's no one else to blame." That line shows how heavily the hurt sits on them.
The mood is quiet, sad, and a little raw. The simple rhymes and repeated "I'm sorry" and "I wanted" make it sound like someone talking themselves through the pain, with no fancy language to hide behind. The ending lands hardest. Even after all these words, the speaker is "too scared / To say this to your face." The poem is the apology, and it may never be delivered.