When a person is depressed they usually get pessimistic thoughts which in turn affects their confidence level and self esteem. And it becomes difficult for them to concentrate on their daily shores. You say u are 17yrs now. If at all you had dyslexia it would have been picked up earlier by your teachers. So currently with the given information I feel you just only having symptoms of depression. The severity and need for the medication needs to be assessed further. Once that is dealt you can look into your issues related to studies.

Hello dear Dyslexia is a learning disorder which includes difficulty in reading, writing, understanding and learning. It is important to be diagnosed by a mental health professional. You might be feeling low due to difficulty in learning and therefore not able to concentrate on studies. Kindly consult a psychiatrist or a psychologist to diagnose the problem and then plan its treatment.