Just got waitlisted after an interview

... I thought I would get in for sure given that my interviewer clearly told me that he was going to recommend me and that I would make a great fit for sure. This kind of bums me out for my other upcoming interviews (given that I rocked the interview yet it still wasn't enough) How large a part does the interview play in the application?
I have an upcoming HBS interview but now I'm worried that regardless of how I do, it wont make that big of a difference....Any helpful advice?