J.Z.
I got pregnant with my first when I was 22. Not TOO young, but I was not married and I went from going to clubs with my girlfriends and hanging out with my boyfriend to being pregnant and having to grow up pretty quick. I completely understand how you are feeling. I basically lost all of my friends because we just had total different lives. We couldn't relate anymore. Thank God for my mom! We have always had a good relationship, but ever since then she has been my best friend. If I were you I would try to find a local mom's group (preferrably for young mom's) even if you find an internet support group, looking back I think that really would have helped me. It always helps when you can talk with someone else who is going through the same thing as you.
As far as your boyfriend...Only time will tell. It is OBVIOUSLY harder on you because you have to grow up over night and no matter how much you tell him that he is not going to "get it". All I can say is give him time, but don't put up with it for too long. I imagine that he is the first of his friends to be a Dad, so he has nobody to use as an example of how he should be acting. My husband and I had some bumps in the road those first couple of years, but we now have four kids (oldest one 8) and I couldn't live without him.
Just focus on the baby. If you have any relatives like your mom or an aunt, lean on them!! And research some young mom groups even if it is on the internet. It will help so much with the loneliness. Good luck and hang in there!