Cough syrup tends to have a bad taste, but once you get it down, it makes you feel better. I think the narrator is facing a lot of problems/difficulties in life, but he knows that once he gets through them (or the "cough syrup come[s] down"), life will be better; he will feel better. Essentially, life's too short to worry about the bad times when they'll always be good times that follow.
Cough syrup tends to have a bad taste, but once you get it down, it makes you feel better. I think the narrator is facing a lot of problems/difficulties in life, but he knows that once he gets through them (or the "cough syrup come[s] down"), life will be better; he will feel better. Essentially, life's too short to worry about the bad times when they'll always be good times that follow.