Practise, practise, practise. Most of the marks in A-Level Physics come from your ability to apply your subject knowledge to a problem and work towards an answer. Particle Physics you can just learn your stuff and be done with it, but telling me that V = IR and Ek = 1/2mv^2 will not give you marks; you have to take that knowledge - and your knowledge of the rest of the course - and apply it to a novel situation.
The best way to improve on that? Do loads of questions. Revision guides, textbooks, and past papers are all helpful, and if you ask your teachers I'm sure that they will have something stashed away that you can do as extra work.
Physics is a really interesting subject, but its reputation for being awkward and difficult is not necessarily unfounded. Once you've done that, though, you'll start to think about things in different ways, and an ability to solve difficult problems where the solution is not necessarily immediately obvious will be beneficial for you in any capacity - good luck!