I was in a similar situation in year 12 where I got a D in my psychology mock but when doing my A-level I got an A. For context psychology a level is 3 paper and on the third, which is the hardest I got an A*. So yes it is possible but it took me a lot of efforts and work to get to this point. Honestly I wouldn’t resit, in year 12 I wanted to but I don’t think it’s worth it. If you’re considering resisting because of UCAS point and universities, then realise that your predicted grades are not only based on the grades you had in your mocks but in other circumstances as well. Honestly I would recommend not resisting and use this time to focus, identify your weakness in each subjects and how to improve, there are plenty of videos on YouTube, then come back stronger in year 13.