I was also what you would call a "highly-functioning alcoholic." I worked ten times harder to maintain a somewhat normal life so I could show myself (and everyone else) that drinking was not interfering with my life. I knew that if I started missing work then I would have to stop drinking.
Highly functioning though? Yeah right. Normal people don't have a splitting headache 360 days a year. They don't spend hundreds of dollars a month of something totally wasteful. They don't really on a substance to feel normal.
I think highly functional people do things like join softball leagues, try to get 8 hours of sleep a night and eat decently healthy food. Highly functional people don't need a bottle of vodka to watch a movie. They can go out and drive places after 8 pm, have fun, and the drive home safely at 11 pm.
I thought I was highly functional but I wasn't.