Im sorry you feel this way.
I feel this way a little bit too. I missed out on my high school years, I had to finish school at home. Somedays I wish I could have been in HS...but it's okay. I'm glad I was able to stay home. But I often wonder what it would have been like if I had stayed in school. I didn't have any friends either (and still don't, besides some online)
Just because you turn 18, and become an adult, doesn't mean you are finished growing...you still are learning, and changing as a person. My mom told me once that her young adult years, 20s, were the best years of her life because she was still young.
College, if you go, can be a different experience, it's probably not like HS, maybe it can be different in a good way for you.
There can be things to look forward to. I know that's really hard with depression though...but there is always hope, and you can look forward to brighter days. You are not lonely forever, that is just a temporary thought/feeling.
I know what you mean having the fun part of your life going away, but when you are an adult, they can be fun too, even with more responsibilities. We only live 18 years as a "kid"...if a person lives until they are 80, that is 62 more years of being an adult...life can't just be all full of worries and sorrows and dullness!
You have only been to therapy twice. Try to have patience and hang in there. There is still time.
I don't know how much you like to read or if you would even like this book, but I recently read this one book called "Monochrome Days: A First-Hand Account of One Teenager's Experience With Depression by Cait Irwin" Maybe you would like to look into it. It was a pretty good book.