Impostor syndrome can be defined as a collection of feelings of inadequacy that persist despite evident success. It is that nagging feeling that the admissions committee made a mistake, and you don't deserve to be at Harvard. See this HBR post. Well, you're not alone. Some, if not many, students feel this way at some point in their academic career. Let us assure you, you deserve to be here. Best way to cope with these feelings is to talk to your peers and make friends. Feeling connected to the community will help make you feel like you belong here.
Reaching out is important. If you have a problem, let people know, make use of the counseling resources available to you. If you need someone to talk to privately, please see the Health Services resources on Counseling and Mental Health.