I can totally relate. My girlfriends are the type who want to go out and get drunk every weekend, which invariably results in a post-drinking fast food fest and then crappy hangover food the next day. I only ever join them about once a month or so, and even though I feel so guilty when ordering fast food at three in the morning I can't stop myself. It's a combination of drunken wanting and peer pressure, I think.
I'm thinking of giving up alcohol altogether (apart from the odd glass of wine or cider, but will stop at one) for the next few months and hoping it helps to improve my healthy eating habits.