"Why do people not believe us if we have medical problems? "
Because more than a few people like to claim medical problems or disabilities, so that they don't' have to be accountable for their actions and/or get benefits that otherwise wouldn't be available to them. More people have caught on to this, and the public at large has been having less and less tolerance for it especially since the economy has gone down.
"Why do people who are to run housing for people with disabilities seem to pick which people to serve?"
Different places can best accommodate different disabilities. Some places are better equipped to have people who have physical disabilities, some specialize with people with mental disabilities. And some places are only prepared to handle people with moderate disabilities.
"How do we be positive in such a negative & unfair world? "
Life's not fair, never has been, never will be.
"3 letters :'God'"
If you want to let it all roll on your deity of choice, that's fine with me. I'll take a more proactive approach.