Thank you for writing. Although I can’t make a diagnosis on the basis of a letter, I can tell you that the feelings you describe are consistent with depression. As you probably know, people with high functioning autism are often anxious. The isolation due to difficulties with social interactions and communication can also trigger feelings of depression.
If you are not already in therapy, I hope you will consider seeing someone for an evaluation and recommendations. Look for a therapist who is experienced in working with teens on the autism spectrum. Such a therapist will help you make sense of your feelings and also learn some compensatory behaviors so you can be more comfortable with other people.
I also suggest that you read some of Temple Grandin’s books. She is an articulate and helpful member of the community of people on the spectrum. I think you would find her story helpful. There is also an excellent made for TV movie that was filmed in 2010 about her life. Do check out her website as well: http://templegrandin.com/. I think her articles will be very reasssuring.
I wish you well.