Embarrassed myself again

Just how can you study 12 hours of French a week for 33 weeks and still not know what the supermarket staff are saying to you?
It's always the same. Approach checkout nervously because I just know I'm going to make a fool of myself. I'm ready with a smile and a Bonne Journee which I will use at the drop of a hat, all the time praying they are not going to ask me any questions. Then it happens. Bag-filling man says something. I look blank. Pardon? (with French accent). He says it again. It sounds like one word but I'm guessing it's probably eight rolled into one. I hate that my French is so bad. Then I blow it completely and speak to him in English. This is bad for two reasons - it proves that I am hopeless at French but he is now thinking, oh another Anglo who has never bothered to learn our official language. What I want to say is hey mate, I've only been here a couple of years. I'm really trying. I've been studying for 33 weeks!
But what do I do? Another Bonne Journee, a smile and a sheepish exit.