I remember a time when I felt like that. It was during my attempt at college, where everyone was so much younger than I was. I felt like an old coot and that I didn't belong.
Please take comfort that you are definitely not alone here. Use this site. Play the games. Join the groups. Post your thoughts and feelings wherever and whenever you can to get out those feelings.
I am currently feeling lost, like I don't belong anywhere and I found a place here. But if there is a counselor at your school it would be a good idea to talk to him/her because it really could help, you just have to let people in and give them a chance. Its easier said than done for people like us. I know for me the bi-polar can either make me the life of the party or the outcast in the corner.
It's tough but you're not alone.