Being "high functional"

I've always been described as being "high functional". I was diagnosed with severe clinical depression, but most people wouldn't know it. I maintain a relatively normal and busy life, regularly socialize, and almost always appear happy. In fact most people say they think I am one of the happiest people they know. I guess that's because I've always been very good at compartmentalization. If I feel like my emotions are going to get in the way, I just kind of put them away in a box somewhere on a shelf in the back of my mind were they slowly build pressure and eventually explode. But that usually only happens when I am alone. Alone, my true self comes out. All the sadness, the hopelessness, the self hate. But when I am around people, it's like I wear a perfect mask. It's automatic, I don't even try to put it on. I can't help it either. Sometimes I want to let myself out in front of others, sometimes I could use the support. But it just won't come off. It's so constantly there that sometimes I use it as a coping mechanism. Like if I'm feeling really bad and I feel like I might be a danger, I go out and hang out with my friends. I put on that mask and pretend to be happy. And sometimes I eventually believe it.