The problem word there is "fake." Why do you have to fake? Be real, if you are sad, that is not a character defect. If someone asks how are you, or you seem sad, if they are truly asking because they care you can tell them as much as you want and perhaps they will help you. If it's someone who is not really concerned but just making conversation, just tell them you are having a bad day and thanks for asking. It is not a sin to be sad.