The poem seems to be about self-harm, and the "TW" in the subtitle is a trigger warning for exactly that. The speaker fixates on a nearly dried-out marker and imagines its black ink running through their veins. Drawing on skin with ink appears to be a substitute for cutting, something that "makes up for the lost blood" without actually drawing any.
The feeling underneath is exhausted and numb. The speaker describes a night spent gasping after a whole day of begging for tears that never came, which suggests pain with no release. The "bitter resolve" and the decision they repeat, "I did make a decision," read like someone trying to convince themselves they are managing. The closing line, "I will let this be enough, I'm still in control," sounds less like confidence than like a fragile promise.
The mood is tense and quietly sad. The looping, hesitant wording ("sharpie ink pen or maybe it's a marker") makes the speaker seem stuck, circling the same thought while holding on to a small act of restraint.