She can discern that she's different from other kids her age. She's expressing this in a way that's heartbreaking for me to even read, let alone to deal with as a parent. As hard as it is, the time has come to validate her suspicions. If she didn't need some sort of validation, she wouldn't be asking you. At the risk of sounding chiché-ish, it's time for her to live with Autism Spectrum Disorder as part of who she is and not as something to be afraid of. Be aware, though, that this is a difficult time for kids to hear that they are different.
There are famous and successful people with ASD, for example, look at the immense relief of animal suffering thanks to Temple Grandin. Susan Boyle has blessed the world with her beautiful voice. (If you've never seen her Britain's Got Talent debut, you really should; it's a bit painful but it's a huge -and tear-inducing - wake-up call.) There is so much awareness of and support for ASD that the diagnosis no longer carries the stigma it once did, and help dealing with issues such as this one is not difficult to find.
If you're not familiar with Autism Speaks or autisticadvocacy.org, please visit the sites. There is a wealth of information for families, including for adolescents. I don't know what kind of therapy she's been receiving, but there is a ton of evidence that treatment helps with social skills, etc. The journal Autism may be available by request at your library. It's a great source of information, including dealing with issues such as this one.
[T]he possibility of problems occurring is more likely when someone is not told about their disability and given the support they need... Not understanding others or social situations for many leads to poor interactions with others and results in ridicule and isolation... You can look for the presence of certain signs that the child is ready for information. Some children will actually ask, “What is wrong with me?,” “Why can’t I be like everybody else?,” “Why can’t I _____?,” or even “What is wrong with everyone?” These types of questions are certainly a clear indication that they need some information about their diagnosis. - Getting Started: Introducing Your Child to His or Her Diagnosis of Autism or Asperger Syndrome
I would recommend you read the source above.
You may want to break the information up into parts, and tell your child that this is going to be ongoing dialogue. (If your child had a newly diagnosed cardiac problem, wouldn't this also be true?) What is your daughter's preferred learning style? If she likes books, many are available for you and that you might read a book together, such as (among others) Freaks, Geeks & Asperger Syndrome: A User Guide to Adolescence written by a 13 year old male with Asperger’s, who was able to address problems kids with this diagnosis commonly face. If she likes movies and videos, there's no shortage of these either. The talk needs to come first, though, before the exploration.
These aren't my usual scholarly-type recommendations, but they address your question.
Top 10 Autism Websites Recommended by Parents
Parent Tips: "You Have Autism."
Your child has autism. How (and when) do you tell him?
10 Guidelines for Telling Your Child about ASD