Definitely go to your teachers and ask for help straight away, usually with chemistry when you don’t understand one lesson you don’t understand the next and it becomes a whole chapter you don’t understand. Definitely do past papers and you could watch some YouTube videos on the stuff you’re doing at the moment. Try to read ahead too so you will have already seen the content so won’t get so thrown by it. There’s definitely still time to improve your grade if you address the issues you have now.