Nobody wants my presence,
Have you discovered just how many users there are in the world? I've had similar since developing mobility problems. People want to do what they want to do, they don't want to make allowance for someone who is less capable and if you can't keep up you're dropped. I'm sorry. The world can be a brutal place and people are usually only kind on the surface, it stops at the point where they have to put themselves out at all.
On the subject of organisations struggling to find volunteers, I sometimes think they're going about it the wrong way. Looking into it once, I found pages and pages of what were essentially job vacancies on one website for local volunteering opportunities. The positions required several interviews and I just didn't see the point. There was no sense of being glad people wanted to volunteer, no sense of finding out what a person's skills were and finding something they can do to help. It was very much a list of demands they had coupled with the expectation that after you'd jumped through hoops to prove yourself capable you'd give your time and energy for free. I don't know why anyone would apply under those circumstances, those positions should have been paid employment. If companies were more open and welcoming, appreciative of the fact people need to fit their volunteering in around their jobs, families or health needs, then perhaps they wouldn't have such trouble finding volunteers.
OP, I think you're right, a lot of it is jealousy. You've discovered how it feels to be not an average person, someone on the fringes of society. I got the same raised eyebrows and feeling that I wasn't a proper part of society when I didn't work due to health issues. For a start people think they can judge your health issues, their severity, your family circumstances and how big your bank balance is just by looking at you. They almost always are openly envious of the free time they think you have because you don't work and have an assumption that you're always available for whatever they want. There's no concept that your time may be taken up with other things, that stopping work may have been a conscious choice and necessary. Often they'll act as though you should apologize for being "lucky" you don't work. Even if someone is lucky eg a lottery win, why should that be sneered at instead of celebrated? Some people can't be happy for others, it's not in their nature. So I get it. "What do you do?" moves from being judged on (and sometimes dismissed as not valuable because of) your job, to being judged for existing as a human. The former is bad enough, an awful lot of people have jobs not careers and aren't defining their indentity whatsoever on "what they do", but the judgement for not working can be ten times worse. Most people work. If you don't you're an outsider regardless of the reason, it's not always a nice place to be.