Emotional negativity is mediated by neurotransmitters that are continuously degraded by enzymes in the brain, causing the negative emotional state to dissipate if not continuously restimulated.
"negativity isn't just a feeling; it's physical brain chemistry in the form of neurotransmitters. These things don't stick around forever. Enzymes are constantly degrading these things, which means as long as you don't create more by indulging in the chaos of mental theater and distract yourself with something neutral, these bad boy neurotransmitters will quickly get broken down and leave you alone." (said at 0:10:15)
No published record matching the claim that emotional negativity directly equates to specific neurotransmitters whose enzymatic degradation dissipates the emotion within a specific time window unless restimulated was located; this does not prove the claim false. While neurotransmitters (such as monoamines, acetylcholine, and amino acid transmitters) are cleared from synapses rapidly via enzymatic degradation (e.g., monoamine oxidase, catechol-O-methyltransferase) and reuptake transporters on sub-second to minute scales, emotional experiences involve complex, distributed neural circuits, neuroendocrine cascades (such as the hypothalamic-pituitary-adrenal axis), and cognitive processes rather than a direct 1:1 reflection of individual neurotransmitter half-lives.