The speaker is a teenager in love with someone three years older, and the poem is her attempt to talk herself out of it. The first three lines are almost like a list of facts: she was 15 when she met this person, 15 when she fell, and they were 18. The repetition makes the age gap feel like the one thing she can't get past.
The fourth line is the heart of it. She admits she'll never be looked at as someone who drives the other person "crazy." Instead, she's looked at the way you'd look at a toddler saying a first word, with fondness but nothing romantic. That image is both tender and painful.
The ending swings between pleading and self-defense. "Please love me" is raw, but she says she'd be "over you in a week," as if shrinking her feelings could protect her. Calling herself "only a kid" sounds half like an excuse and half like she's trying to believe it. The mood is wistful, vulnerable, and quietly embarrassed.