The speaker feels stuck inside their own mind, with thoughts and desires they're too afraid to share in case people laugh at them. The fear of ridicule keeps everything shut in, and the poem treats that silence like a prison. The speaker says they feel like a prisoner, "always locked away."
There's a sad twist in the line about being authentic only "under lock and key." The speaker seems to be their truest self only when hidden, which suggests that being seen feels unsafe. Holding everything in doesn't stay harmless either: it "spreads throughout my body" and starts to hurt, so the strain feels physical as well as emotional.
The image of letting thoughts out "droplet at a time" shows a careful, difficult effort at opening up. The ending suggests writing is the outlet, a way to keep from "spilling." The mood is anxious, lonely and heavy, though the act of writing the poem hints at a small, quiet step toward release.