TL;DR: directly jump to part 3, pal!
Disclaimer: I am autistic, I had my diagnosis at the end of 2018.
Now that being said:
Part 1: Autism can be a handicap
I know I'm gonna have some teeth-grinding here, so let me explain.
Autism is not a mental illness, nor it is a mental health issue, although it is called "autism spectrum disorder" in English, and there's a lot of associations that rather focus on the relatives' discomfort of having a child, sibling or parent that is autistic than on the well-being of autistic people.
To prove my point here, let me cite this brilliant article written by Devon Price on Medium about autism and neurotypical people's difficulty to accept autism*:
"A lot of folks want Autism — and, obviously, all Autistic people — eradicated. Autism Speaks describes Autism as an epidemic and a disorder to “cure”. The state of Colorado is considering declaration of an Autism Epidemic Emergency. Our existence is that menacing to people. We are seen as that incapable of living worthwhile lives. Like an addiction, a cancer, or a progressive, deadly disease, Autism is assumed to be a malignant thing that no person could ever want. A lot of parents, doctors, and psychologists would prefer that Autism be screened for during pregnancy, just like Down’s Syndrome, so that parents could systematically remove us from existence."
*note: The emphazed pieces represent two links to "Autism Speaks" news that I did not copied because they're now broken, as the association decided to remove those articles from their website.
This other article from Medium on the possibility of "curing autism with gut bacteria" is a good example of the stigma that remains towards autistic people and the idea that they are sick.
Autism is not a disease. No matter what some people may say about it, it's not. Just embrace this. So what makes it become a handicap, then? It is that in many countries around the world, autistic people specificities are not taken into account into how the society works.
An example: many autistic people have higher sensitivity to sounds, sights and smells. I myself am very sensitive to light and sounds. Now picture me going to the grocery store: I have to face the bright neons, the speakers continuously shouting commercials and music, people yelling at each other about which tomato sauce to buy or to not make a scene in front of everyone because "Daddy doesn't want to buy you this candy". Grocery shopping can be exhausting for many autistic people. And this is where the handicap may exist: for autistic people to evolve in a society that is not well-suited for their needs and specificities.
In Atypical, a series picturing daily scenes of a high school autistic boy, there's a scene in which his girlfriend is trying to convince the school to organize a Silent Prom. The idea is to play the music into headphones for whoever wants to enjoy the music and let those who would rather not listen to it enjoy the party without it. This is a brilliant example of accommodation to improve the inclusion of autistic people in the society. The thing is, the school and students parents strongly disapprove the idea, for "it won't be the same as a real prom for our kids who are not autistic".
The conclusion here is: autism is nothing of a disease, although the term "Autism Spectrum Disorder" is confusing. But autism can be a handicap, in the cases where the public spaces, jobs and workplaces, and society in general are thought to meet neurotypical needs and specificities only.
Part 2: The specificities of being autistic in France
For people living in France (OP and I are from the same country), they are two types of diagnosis: most of the time, a healthcare practician diagnoses autism (a psychologist, a psychiatrist or a physician, although this is quite rare), then a specialized centre called "Centre for Autistic Resources" (CAR) has to study your case and validate the diagnosis, so that you can be officially (i.e., by the country and your employer) recognized as autistic. Now, although the healthcare diagnosis is reliable and acknowledges that you have been recognized as autistic by specialists of autism, this is not enough to be recognized as an autistic person in many situations. For instance, in France you need to have that proof from the CAR that you're autistic to benefit from working accommodation, like a different job timetable, a possibility to isolate yourself, or even the right to wear headphones while at the office to reduce the surrounding noise (some French companies forbid to their employees to wear headphones at work).
The issue with that double diagnosis is that, upon the few people that are a bit aware of autism, most of them do not believe you until you get that proof from the CAR. And it takes no less than three years to even get an appointment at one of those centers. So most of the time, you get a diagnosis from a healthcare practician, "yay, congrats, you're autistic", but you don't get any help to improve your inclusion into society. In Europe, in 2014, 76% to 90% of autistic people were unemployed (English reference here). I cannot find the numbers for France but last time I saw them, it was above 90%. In France, many autistic people are even forced to exile themselves in more clement countries, like Belgium.
All of this to say:
it's difficult being autistic in France, as there is a real stigma against autism and most people know almost nothing about it.
Part 3: How to answer to people who do not believe you
Regarding all the information given above, it sounds obvious that you want to be careful on how to talk about your autism to people, whether you're close to them or not.
The next time someone tells you "but you don't look autistic!", I suggest you answer something along the lines of:
That's funny you know, there's a saying in the autistic community: "if you met someone autistic, you just met one person with autism". There's as many forms of autism as autistic people. My specificities are unique, some of them may be visible, some may be not.
This is a friendly reminder that the manifestation of your autism may not be as obvious as they would think, and it implies that you do not want to have to justify yourself. I think that at least in the first place, it's important to remain neutral in your tone and not convey they hurt you because as many answers already suggested, I believe that most of the time, when people say "I can't believe you're autistic" they don't mean harm, they just have very little knowledge of autism. If they insist, you can invite them to learn about it:
As I said, autistic people are all different. If you want to know more about it, there's a lot of articles and websites depicting some of the most common specificities. [I could send you some of them, if you want].
This reinforces the fact that you don't want to explain makes you an autistic person. By then, they should have understand that you don't want to talk about it. Add the last sentence if you feel that you're close enough to your interlocutor and that they may be interested about it.
Part 4: Keeping up
Now some advice from a gal who came out in front of all her colleagues in the middle of a fight on whether we should rearrange our offices to build an open space:
- Keep in mind that most of the time, the people asking do not mean harm. They just don't know a lot about autism. It will help people answering their questions. If you feel overwhelmed by their questions, invite them to learn more about autism online. Give them a few links if you feel that they would definitely read what you send them.
- Don't feel bad for who you are. You're not more or less, you're just different.
- Get closer to people on the spectrum, join online discussion groups, share your tips and troubles with them. It will also make you feel understood and loved for who you are.
- A tricky one: do not come out when unnecessary, yet do not try to hide your autism. "What the heck are you saying, Ava?" I'm saying that the word "autism" comes with a lot of prejudices. I think that the issue people have is not with the way you are or behave. It's with the word "autism" itself. It's exhausting having to justify yourself about your diagnosis. But it's also exhausting trying to hide who you are and how you behave. Before discovering that I'm autistic, I've been labeled as "weird". I was the weird girl, the kind, generous, funny, but weird girl. If you're fine with that label, I think it's better than to be thought of as "the person who tries to come up with a """disease""" just because she's weird". It may be a coward move (not disclosing your autism), I agree, and I don't know about you, but I don't have the strength to advocate for autism awareness to everyone I know. Instead, I anonymously write post blogs about autism.
You're doing a great job so, keep going!