Solitude is the hero here. The speaker says that when they are alone, their thoughts feel "grown" and "strong," as if being by themselves lets their mind reach its full size. Alone, they feel "the truth / Of being me," a plain, confident statement of knowing who they are.
The second half sets this against time spent with other people. Around them, the speaker feels they have to keep their thoughts "mute," as though holding back what they really think. The people are called "not real," which probably doesn't mean they don't exist. Judging by the context, it suggests that these relationships feel fake or surface-level, with no room for honesty.
The mood is quiet, calm and a little defiant. The short lines and simple rhymes ("grown" and "alone," "mute" and "to") give it a steady, almost chant-like feel. The trailing dots at the end leave a lingering sense of loneliness mixed with relief.