Let me tell you my story. So I didn't do so well in my A-Levels after year 13 so I decided to do a third year of A-Levels to resit some exams. I achieved a grade D in biology in year 12 after AS levels and a D again in year 13 after A2's. Although by the end of year 13, my AS biology did improve to a C. My teacher needed to give me predicted grades for UCAS, I begged for a B grade prediction. She thought it was unlikely for me to achieve that and said the best I could do in A-Level Biology is to get a C grade. Never the less, she predicted me a B in the end. Along came results day, I had actually improved my D grade in A-Level biology to an A grade. I had retaken all my biology modules (AS+A2) again.
This goes to show that nothing is impossible. Put in the hard work and don't be put down by them teachers. Prove them wrong! Shame I didn't get to see my biology teachers after results day, I would have love to have seen their reaction. I honestly did feel really useless in Biology but I suppose it only made me more motivated to prove them wrong. The same goes to you with Chemistry, work hard and you can do it!