I'm very open about my son's health issues - because I truly believe it shouldn't be treated any different than if he had diabetes or cancer or anything else.
And, I find that many people have a story and do understand.
I don't always pop out the SZ word at first, and he's not officially diagnosed with that anyway, but I'll say he has health problems, then if they ask more, I'll explain. People are usually more comfortable with psychotic depression (he does get diagnosed with that now & then) or severe bipolar (what I think he might really have), but if someone else says SZ, then I'll say that too (because that could be it too - people have mentioned it, but no pdoc will write it down.)
The story I have that relates most to you is with my hairdresser. I recently saw my old hairdresser (she had quit to have kids) in a salon we go to and she asked about my son - she cut his hair when he was a pre-schooler, early elementary.
I started to explain, and she said, I know just what you mean. My mom had really bad bipolar and when we went to the mall as kids, we never knew if she was going to buy something or start to strip off all her clothes.
Same with a nurse practitioner we're very friendly with at my husband's orthopedic doctor's office. She was practically crying one day and said she knew us so well she felt she could talk about it. Her mother had just died and she went into how hard life had been with her because of untreated bipolar. She didn't know about our son at the time, but we went into it a little bit so she'd be more comfortable.
Just wanted to say you're not alone, but people don't know who they can talk about it with. I'm betting you come across people everyday with family members with severe mental illness & you just don't know it.
The PC word now is brain disorder, but I almost feel like I'm lying when I say it, but I'm trying.