I think that the song means exactly what it says. A brompton cocktail is a mix of cocaine, morphine and heroine given to terminally ill patients, so they die happy and pain free! And they are looking back on their and the mistakes they made. But on the other hand, it may be a metaphor for him needing something similar to a brompton cocktail (not drugs) in other to make him stop looking back, and to enjoy the rest of his days.