You are scared of exams because exams are scary for almost everyone. It’s absolutely normal to be stressed. It’s not normal to be so stressed that you get depressed and lose confidence in yourself to this extent. And, no, I don’t think you are expecting too much of yourself. If you want to be a doctor, you need to study and master difficult subjects. That is challenging for anyone. It is also worth doing.
I have several suggestions:
1) Please go back to the doctor who prescribed the Lexapro. Talk about whether it is time to discontinue it or to make a change in dosage or in medication.
2) Then, please, find a therapist. Anxiety and depression are rarely taken care of by medication alone. You need to learn some new coping skills for dealing with stress. Cognitive-behavioral therapy is generally thought to be the best approach for something like this.
3) Talk to someone about whether you have the study skills you need to be the kind of student you want to be. There may be more effective ways to study that will give you more confidence when you are faced with an exam. Make sure you allow plenty of time before the exam to study. Many people find it helpful to have a study buddy; someone who is taking the same subject so you can quiz each other.
4) Make sure you are taking good care of yourself. Students who eat right, get enough sleep, and exercise regularly do better on their exams.
5) One final hint: Believe it or not, people who get a good night’s sleep the night before, get up early, eat a good breakfast and dress nicely generally do better on exams.
I wish you well.