Definitely take what the guy above said seriously. Since you missed out on a lot, I looks like some fundamental knowledge hasn't been absorbed as it should have. Explain to your teacher exactly what you don't understand, and DON'T BE EMBARASSED! Don't just nod your head even though you don't get why something is the way it is, even the smallest bits. I was guilty of pretending to understand myself, and I can tell you that it only harms yourself. you said that he might think you're a bother but honestly, if you seek his help and improve, I am certain that he won't mind in the end.
I know this sounds like a mean thing to do, but maybe see what the average in your class is, especially if you go to a school that gives harder tests. My teacher put our class average on the program we use so that parents wouldn't freak out about their child's low score, it was 58%.
Have you considered making a study group? Talking with other students is also a very big help. Even online, on discord or even Reddit there are communities ready for you to join. Good luck.