I too, understand what you are talking about. Sometimes I feel isolated because my friends and family cannot relate. I feel like a burden, because everyone else lives seemingly perfect lives and I'm the nut job who has meltdowns. I too find it difficult to talk about my past and my feelings, mostly because vulnerability is an uncomfortable contradiction of the wall that is always up. Although I wish they understood me, I am certainly glad, they don't know PTSD like I do.
Sometimes I cry out of frustration with myself. I cry because my family assumes my PTSD is a phase, that I will be normal again soon. I cry because my efforts seem pointless. I cry because I miss having normal relationships. What I'm saying is, it's ok to cry. It's ok to do things at your pace, we all have to. And if it means missing a party or ruining plans, so be it. Don't beat yourself up about it.