The speaker is a young person frustrated by mixed messages from an adult. Sometimes they're told they're old enough to understand something; other times they're told they're too young. The poem asks the adult to pick one. If they really think the speaker can understand, they should say so and mean it, and if not, they shouldn't pretend.
The mood is irritated but also a little tired and hurt. The image of walking "on a thin sheet of ice" every time someone speaks captures how tense and careful the speaker has to be, as if any word could crack something. The plain, short lines sound like someone speaking bluntly, losing patience. The last line is the sharpest: being "too young to be myself" suggests the speaker feels that being treated as both child and almost-adult leaves no room to simply be who they are.