Closets can be cozy, and above all safe. There's no shame in staying in one if that's what you're more safe and/or comfortable doing. If you think coming out would do more harm than it would good, my advice would be to stay in the closet until you can move to a more progressive area.

That being said, if you do want to come out, there's no reason you would have to come out to everyone at once. Telling a trusted friend or two is a good place to start. And honestly, my opinion is that if someone doesn't want to be friends with you after you've trusted them with something like that, they're not worth your time anyway. Having no friends is better than having bad friends, in my experience. But I understand that not everybody feels that way, and there can be consequences from coming out to the wrong people.

Regarding the understanding thing... You're right in that people probably won't understand right away. I'm transgender, and I got a lot of uncomfortable questions when I first came out. I still get them sometimes. But again, if they're friends who are worth any merit at all, they'll listen and want to understand you. Granted, people generally don't know as much about demisexuality compared to other LGBTQIA+ identities, but that also means you probably won't have to deal with any incorrect preconceptions or stereotypes your friends might have about it.

Anyway, that's my advice, if it helps. Mostly just only do what you feel comfortable and safe doing. And good luck!!