This is coming from someone who's just finished Year 13, but I definitely think it's possible, it just depends on your drive and motivation. Rather than watching videos on changing your mindset, I think you should utilise this summer to ensure you have solid knowledge on your Year 12 content - if you don't, you will struggle next year. This also heavily depends on what subjects you do. For example, if you take Maths, I would definitely recommend utilising your textbook, online videos and past paper questions to get familiar with the subject. But if you take essay subjects, the way you approach it would be entirely different - I would instead suggest trying to make notes or revision materials on each topic i.e. for a subject like Politics, learn how your papers are split up and how many topics are in each of them (you can do this by referring to the spec) and then create revision materials from there. A-Levels is a game which only looks for the end result, it doesn't really care how you go about achieving it. My best suggestion for the you who is in this situation is to come up with a plan of how you will be tackling your three subjects. Concentrate most of your time into turning your C into a B, and your E into a C. You CAN do this, you just need to work extremely hard for this!
Edit: I will preface that if you're looking for BBC as your predicted grades for UCAS, this will most likely be impossible unless you re-take the year!
I hope this helps, feel free to reach out if you need any help!