Keep your head up! Physics is a subject you may spend ages on and understand nothing. Luckily, the grade boundaries are quite low because this is the case with everyone. If you're doing well with your other 3 subjects though, I recommend trying to focus on Physics a bit more in terms of revision, as it could make all the difference. For a safe A, I recommend you try for 75% because in most cases it should be an A*, but what Kyaurix said is scaring me so just in case xx
But hey, practice makes perfect, so good luck!