I got diagnosed at 18 in person. It’s not as stressful as you think. Over 50% of it was taking little online quizzes, and part was doing different puzzles, and then you sit with a person and talk for a while. I’d make sure to bring a water and wear whatever clothes give you the least sensory issues. But, it’s a pretty positive experience.
You probably won’t be dismissed. I think the hardest part was the talking, cause I’m not the most social, but the professionals are trained to know what to do. And if you are uncomfortable? There’s no shame in letting them know. The white noise machine in the room was irritating me so the test person shut it off. But, anyways, good luck to you!! You’ve got this, and I hope you find some validation.