....at what age did your life improve? Or is my quality of life only going to diminish as I age?
I know there is no simple answer as we are all different, but it would be interesting to hear from the older generation as to how their lives have panned out through the years.
Even though I am making a conscious effort to view the world in a more positive light now, I remain rather scared that I will be an extremely lonesome and anxious man throughout my life.
For instance, today in a supermarket I believe I managed to offend a female clerk who must have been a similar age to me. I merely enquired to see if the store sold a certain product (melatonin), but she seemed flabbergasted by the way in which I spoke to her.
I am now endeavouring to speak to as many people as possible each day, but I am always afforded with rather quizzical looks. It's difficult for me to pinpoint where I am going wrong.
When one is 27 years old and hasn't got a friend in the world, serious questions need to be asked why this happens to be the case. If I am forever going to be a prisoner inside of my own head, I fear it's inevitable that I will become either an alcoholic or dope addict.
I guess my question is will I ever fit in?

Sorry to say that you won't fit in. Why do you want to? But you'll learn to get along on this strange planet better as you learn better social skills and work on self-acceptance.
Are you happy with who you are? If not, write a list of things that you feel are holding you back. Then find the resources out there that will help you work on those issues. Then get to work.