Understood. If you "got an A* in my end of year 1 mocks" and "thought i was doing ok until my report came back today", then it sounds like you need to talk to your Maths teacher to establish what the issue is. A three-grade drop in prediction from A* in May/June to C now is fairly dramatic!
Do you know the basis upon your new prediction has been established? Have you done a further mock since that at the end of year 1? Did you do significantly worse in this mock? If so, did you have an "off" day, or was it just really hard? Or, conversely, was the end-of-year mock artificially easy?
Or perhaps your new prediction is based upon your work in class, your homework, end-of-topic tests, etc? Are you aware of having struggled in these areas? It sounds like you must have been doing OK, given how much of a surprise this new prediction is.
Either way, I think you need to talk to your Maths teacher to get some specific advice on how to get your self back to an A*.