Please don't think that being 'not fine' is OK and that you have to hide it away. You don't have to share it with every single person you meet, but it's a good idea to share with people you are in regular contact with. Especially your boss. They will then know whats going on the days you seem a little off and won't presume/guess what's going on instead.
You will also feel a weight lifted from the constant need to put up a facade of being positive, happy all the time either which might actually help you to feel a little more positive and happy actually :)
Here for you whenever you need an ear to listen.
I have clinical depression, acute anxiety and an eating disorder. I'm a 37 year old wife to a very understanding husband and Mother to two beautiful children (7 year old and 3 year old). I work full time and have recently discovered that sharing my feelings and illness with my line manager, husband and close friends has took a lot of strain and pressure off.