Hello. I am new here as well and just found your post. You say you wish you were diagnosed sooner and I believe you, but please look at the bright sides, being diagnosed at 15 is a blessing, it is very young and you are part of a newer generation that is being tolerated well in terms of diagnosing and treating ASDs, a diagnosis would be life changing for a lot of undiagnosed adults who struggle to find mental health professionals to take us seriously.
I myself sound a lot like you, but a generation older, in my 20s. I can definitely relate to your symptoms at that age (the hand flapping and other physical quirks were and to some extent still are problems for me), however I remain undiagnosed at 27 and have struggled a lot. It would be great if people like us could have been tested before our parents even enrolled us in school, but while you wish you were diagnosed sooner, I'd have loved to be diagnosed at 15, I only really began researching Asperger's within the last year and it explains a lot about me and my life, to have been diagnosed in high school could have changed my life, so please keep in mind how a young-age diagnosis will shape your future
Like you I have struggled with misdiagnosis, this was especially bad growing up and still living in a small rural town, I have run into a lot of people who were supposed to HELP ME but only saw me for a few sessions and concluded stuff like I 'need to suck it up'...this area has very bad and unprofessional mental health workers who seem scared to deal with things they lack experience in. The only diagnosis I was ever able to get was depression and I stupidly trusted this person's diagnosis, I then got on Wellbutrin which almost made me kill myself because I had been given a medication for a depression that I did not have, rather I merely had depression-like symptoms which stemmed from problems caused undiagnosed Asperger's (still working on finding an experienced specialist who won't be quick to write things off, I will only trust an experienced specialist at this point to make the right call).
I hope you find a lot of people here that you can relate to. I know being 15 and feeling alone with your diagnosis is probably very scary, just keep in mind that you have made progress simply by getting the diagnosis and you are still in high school, you've got a lot of time to discover yourself, tackle your problems and adjust into a life that you can be happy with. I hope my post doesn't sound like it is downplaying you, I am intending it to be encouraging, sometimes I am bad at getting things out properly. What I am saying is that you might perceive your situation as difficult and it most likely is but dealing with these types of problems as early as possible is the best solution. You are coming to terms with it now and finding other people here on WrongPlanet, it would be nice to have been diagnoses at age 6 or something, but being diagnosed at 15 is still way better than being diagnosed 10 years later or never diagnosed.
Do you know about whether or not you have a special interest? Or anything you are very good at compared to your peers? I don't want to sound overwhelming, but it can be good to explore things now so that you don't have problems in college and later in life figuring out what you're good at or what type of job environment would work well for you. I speak from experience because at age 27 I do not have a college degree and I have little work experience that all ended in disasters regarding my social life and reputation, the same problems that have caused me to lose jobs constantly have also given me a lot of trouble in college courses, such that I have attended college two times for two different areas of studies and been forced to drop out both times despite getting good grades and enjoying the work, the social aspects ruined me as well as other things like my sensitivities to light and sound, my ability to get lost in thought which would always cause me to get behind in my studies and have to work much harder to stay afloat than other students, I can go on and on (and going on and on is another major problem that gave me college woes).
I hope I didn't type too much, I just figured it might help if I shared some of my life's story with you, maybe I have said something you can relate to, I want you to be comfortable and feel that you can talk it out.