Overthinking is the whole subject here, and the poem shows how it feels rather than just describing it. The opening repeats "Over think" three times, so the reader gets caught in the same loop as the speaker. The speaker admits it's all they ever do, and that their head is overflowing with "toxic thoughts." The mind has become something that's spilling over and poisoning them.
From there the poem shifts into water. The speaker feels like they're sinking and can't breathe, which suggests the thoughts have turned from an annoyance into something that threatens to overwhelm them. "Just let go" is the quietest, most ambiguous line. It could mean giving in to exhaustion, or it could hint at something darker, and the poem doesn't spell it out. "Drowning" repeats three times, mirroring the opening, before the single word "Gone" sits alone at the end.
The mood is heavy, tired, and bleak. The short lines, the rough punctuation, and the sudden emptiness of the ending make it feel raw and unpolished, like a thought written down in the moment.