You're probably more scared than other kids since you had that bad experience in math in your old school. I think you should ask your parents to take you to the middle school to meet the new teachers and the guidance counselor before school starts. They might be able to show you around and let you know what kind of help you can get (if you need it).
If you don't have time to go before school starts, then just make sure you set up a meeting with the guidance counselor at the start of the year. Tell the guidance counselor about your worries, and check in with her often in the first few days and weeks of school. Tell your math teacher that you really want to do well, and that you might need some extra help in the beginning. That way, the teacher can check your work more often and get you the help you need. If you are in a resource room or learning center, then that teacher will be very helpful, too.
I'm sure you'll do just fine. Good luck!