assuming that you can't live whole life without smiling, which i assume is a pretty safe assumption, i'd suggest you to be yourself and not worry about what girls or anyone else think. if someone doesn't like your smile, god help them; it's not your problem, it's theirs instead. and you are not 100% sure that girls don't like your smile, are you? so, stop making assumptions and being too conscious about your smile, just let your facial muscles do the job.