I know its so hard- people want you to be fake when you don't feel well, that's their problem. Your emotions are yours, and you shouldn't hide them. Depression is a chemical imbalance in your brain. PTSD also causes chemical changes. It's like saying to someone with a broken leg, get up, just walk it off you're fine!
Check out this song by Michael Kiwanuka, called "Love & Hate" (on YT, I'm new so I can't post links yet!)
take care, Forestwalker