It happens. Whether you don't want to "be depressing," or you just don't want to have to explain all the facets of your disorder, "fine" tends to be the preferred response even when things aren't at all. I've started to say things like "I'm hanging in there." It allows the person to whom I'm speaking to ask follow up question if they want, but doesn't put pressure on either of us to go into detail. Do you have a response that you like to use? Let us all know in the comments!