I struggle with math as well. I just am not up to pace with everyone else especially with mental math. I wouldn't stress too much because as long as you get the right answer then no one will even care how much time you took to get it done. But when in doubt just talk to the teacher after class and express that you are struggling.
More often than not they will help you and even give you a little boost just for asking and expressing your concern. Hope that's helps at all. :)
