Why is it so hard for me to talk to people?

I'm 13. And I've been going to public school and Been playing soccer forever! But I only have a small amount of friends. I find it really nerve-racking to go up to someone and start talking to them. Every year I get assigned to a different team and I never end up making any good friends. I feel like the luckiest person ever when someone starts a conversation with me. But it's always because I would just be sitting alone looking like a loser and they felt bad. I never really started a conversation unless it's a friend. But I honestly feel like I can't talk to them about this because they are always making fun of me for my goofy personality. During class im shy and people think of me as a nerd. But when I'm with someone I can comfortly talk to, I don't shut up. Any advice on solving this problem? Sorry for the long read, and thanks!