I'm sitting here playing a game with my family and they're all laughing and having a good time, but it's like I don't find any humor in the things they are laughing about, I don't feel interested or engaged in what we are doing. 

At the same time I have to fake being happy and pleasant or else I'll ruin the vibe and if I let my malcontent show even a bit it will look really off to everyone else abd they'll start asking me "what's wrong? ". 

Such an ironic thing for anyone to ask me that. How do you honestly communicate my complex psychological and social ailments in that situation? You don't, there's no point, and you just have to keep lying and keeping a fake face on and just sit there until it's over and you can be alone again.