I often feel lonely even among friends?

I am 14. I go to some extracurricular activities where m friends and I hag out. Sine I am homeschooled, I do not know of the things from their school which they talk about. I try to make conversation, but after we finish the topic we stop and an awful feeling of loneliness overcomes me. I used to be super happy, with a great social life and a lot of friends. But now I have spiraled down into a pit of unhappiness. I can be in a rooom full of my best friends bt feel allone. I know that they care about me, but I still feel like they, like, feel sorry for me. I am worried for my mental health. I am trying desperately to be happy but I am knocked down constantly by this awful feeling of loneliness. Whats wrong with me and what should I do???