I feel that way to, I think it is a fairly common symptom. To me it's one of the worst, like it can hit me pretty hard if I am just trying to enjoy time with friends or family, I'll just feel detatched and distant like they're all enjoying themselves and I might be trying to act like I am but its not how I truly feel. I feel like I'm just watching from a distance.

Its difficult to explain to people and makes me feel like I piss people off by bringing down their mood with it, hence the act but lately I am not so good at doing that unless I get drunk but that hasn't worked out too well the last couple times. But yeah I kind of understand the feeling.