For some of us, being solitary is the way it is. A range of things is normal, including being fairly solitary :) I've never really had lots of close friends either, but I've also found that there are upsides to this - it did, after all, allow me to move countries without many complications, for example. I did this in my mid-30's. I happen to move to a more introverted country than the US - being somewhat solitary definitely helped with this.
I fully suggest things like joining groups if you have them in your area. I understand that not everywhere has these or not in your interest. Join groups even if you think everyone else will be your grandparent's age! You never know who you might find. If you can't join any groups, try to do something regularly. Do you have a local coffee shop or even a local McDonalds? Go there once a week, fairly regularly, and drink coffee while you sketch. It is a great way to meet people (I took a vacation to Amsterdam where I sketched in coffee shops and other public places - it was wonderful). Plus, art gives me something to talk about :)
Experience as much as reasonably possible, especially while you are young. The only other real group hobby I have is tabletop gaming - I go to a friends house when possible to play overly complicated board games. I've thought about volunteer work, in part to improve my language skills.
Casual friendships are gold. While it is hard to find close friends - casual stuff is easier. The grand thing about these friendships is that when put together, they tend to equal a full friendship. One person might be good for getting your mind off things while the other might like to listen to some people's problems from time to time. I tend to have very few close friendships. Heck, I have only a few casual ones as well, but i'm ok with that. I'm not sure if I'm missing out on things, but I stopped caring.
College will be partially the same and partially different than high school. I have enjoyed being an adult much more than I did being a teenager. I'm now 40 and can honestly say I enjoy life much more now. You can have the same kind of youthful fun when you are an adult. What you find fun might (and probably will) change. It is normal to be afraid of ending high school, by the way. Most of us get at least a bit nervous before life changes when things seem a bit unknown.
One thing that might help your perspective is learning how to frame things in life. For example, before I moved to Norway after marrying the spouse (who is Norwegian), I figured that if things went badly and I wound up back in the US... I'd at least have a good story to tell and a mound of life experience that wouldn't have happened otherwise. I figured the story itself would make me more interesting - at least to my standards. This sort of attitude makes difficult times ever-so-slightly easier and the fear of the upcoming changes much easier. :)
And this might not have been the best writing, but that's all I'm doing for now :)