If your teacher thinks it's possible then there we have it. I guess the only way to know if you personally can cope with it is to try it.
i.e. try and work out how many chapters you'd need to cover a week to catch up in time, try that amount of learning for two weeks, and if you can keep up then there we go!! Don't be shy to ask people who've done science from the beginning for help, too. Having your peers help explain stuff can be really helpful.
If your school does AQA for chemistry then I recommend using the website chemguide alongside textbook. It's not very pretty but it has everything you need to know laid out plain (if your school doesn't do AQA I'm sure it's still useful as they all use the same national curiculum). The guy who made chemguide also has a book called "Calculations for A Level Chemistry" which is a textbook that just covers the maths in chemistry; it's been an absolute lifesaver for me.