Hi Priya,
Don't get disappointed. Believe in yourself and you can definitely get good marks. Firstly,  you should stay calm. Try to do meditation in the morning to stay calm and focused. You can also read some inspirational quotes to stay motivated. See, motivation can comes from within. You are dedicated to get a seat in decent medical college and become a doctor then disappointment and negativity are just hurdles for you. You need to overcome this and score good marks.
Try to keep your concepts clear and have good understanding of the topics. Take your time and try to analyze your mistakes, where you are lacking and then work to improve your mistakes. See, we all learn from our mistakes only.
Whenever you study or appear for any test, concentrate properly and stay calm.
Also, if you  score bad in a test don't keep thinking about the bad scores infact you should work on to improve your scores.
With positive approach, scheduled study plan, determination and dedication, you can surely score good marks.
All the Best!