You're not alone, the disbelief of others is something you need to adapt to because it's not going away anytime soon. It says you're 17, that is about the worst age for someone with autism (not to mention your other diagnoses)-- that's the age when social pressure is amped to it's max, and everyone is about "fitting in". The pressure to "fit in" will always be there, but it will ease down a bit in a few years.
Realize that as harsh as the teenage world you're living in now can be, there are places like this that can provide shelter from the storm. If you have a problem or issue, or need to vent, I highly recommend letting out here as opposed to your classmates. At your age anything you say will only be used against you in that setting. Here however, (and other similar forums, if this isn't your cup of tea) you'll encounter people who have been through the gauntlet you are navigating right now-- they have knowledge and empathy, two qualities your won't find in your classmates right now.
As I said, I can't offer a solution-- if I could change human nature I would, but alas, I don't have a magic wand powerful enough. All I can say is that it will get easier and the pressure will drop a bit, you just have to hang in and wait it out.