Tinkering with brain chemistry may be another impure thought! ;)
I would focus on parasympathetic breathing and mindfulness, and gradually make carefully considered dietary and lifestyle changes.
Low and slow will give you less to undo and allow the brain to rebalance naturally. Your question makes me wonder what interventions you've already tried that have further imbalanced natural balancing mechanisms of which we understand so little.
Why not try some magnesium, lemon balm tea, Rescue Remedy or any number of benign ways to calm racing thoughts in the mean time?