The hardest part with being high functioning is that you mostly are ok to be able to get on with life. And people get to know you like that. When you really are struggling, you need to uphold this perception people have of you, and there is nothing more painful than wearing that mask when inside of you is being torn apart.
People will never believe there is anything wrong with you, because you manage to keep it together most of the time.
You also have a "reputation" to uphold, which justifies your job and keeping you employed. It gives you a certain responsibility you need to stick to. Yes, it gets you out of bed on those days you feel you can't, but you then expend an AWFUL lot of energy on keeping things together. You also mostly aren't prepared to disclose your condition, so again you need to wear that mask when things get tough