The speaker begins with a shrug: they've done plenty of strange things, and none of them bothered them much. Only one thing stuck as a real regret. The title, "A Creep," suggests how the speaker might see themselves, or how others may have seen them, though the poem never says what the regretted act was. That silence keeps the focus on the feeling rather than the event.
The last three lines are where the poem turns. The regret is "fading in the background," but the closing line, "as if I don't exist," leaves it unclear what exactly is fading. It could be the memory, or it could be the speaker, slipping out of other people's notice. Judging by the wording, the speaker may feel erased, and the regret and the loneliness seem tangled together. The mood is quiet, flat, and a little sad. Short lines and plain words make it sound like someone admitting something under their breath, without asking for sympathy.