I am just like you. Everyone knows me as the shy quiet person who "never talked" but the problem was, was that nobody never chanced theirselves talking to me. I'm a rising senior about to enter my last year of highschool. My most recent year, I had really bad social anxiety due to an event that gave me depression. I suggest this:
Try to change your style of clothing- about 90% of how people judge you is on your clothing. Just dress your own way, or if you want to take a turn for a different style, then its up to you-a different hairstyle wouldn't hurt. Next, I know this isn't easy but I realized that in life, you're gonna have to not be so quiet at some point. You're gonna have to talk, but how? Well you can start off easy such as answering teacher's questions in class, group work will help as well. Try to talk to the teachers just normal conversation (trust me i've been a teacher's pet and they LOVE the quiet ones). You can also try joining clubs as well- I just recently joined one in hopes of opening up a bit.
Your past-yup I had a bad one as well, long story short, I used to be the popular kid until i made a mistake. But you have to prove them wrong- everyone only judged you by the cover they haven't read the book yet. Freshen yourself up a bit. We're both in the same situation, so I hope this helps your situation :)