Like Swapna mentioned, this happens to all freshers. But also, don't think that experienced persons are experts in handling interviews. You will be surprised when you know the facts.
Addressing Your Concern
First and foremost, please stop thinking from the interviewer's perspective. This means that if you judge whether a particular answer will satisfy him or her, that's when your problem begins. Every interviewer has his or her own perspective—even if you think you have given the perfect answer, chances are very high that each interviewer will look at it differently and give scores accordingly.
To give an example, you got 66% in your MBA. I may think it's okay, another may think it's a very low percentage, and yet another may rate it differently. You have no way to know what the interviewer is thinking.
Unspoken Communication in Interviews
Another important factor that quite a few people overlook is that much of the interview assessment happens through the unspoken word—basically the body language of the candidate if it's a face-to-face interview, or the tone and texture of the voice if it's a teleconference. It's not what is being said, but how it's being said. That's where Ramu's suggestion—be 100% confident—comes into the picture.
Focus on Your Strengths
From your preparation point of view, I suggest focusing on yourself, your strengths, and maybe limitations (I wouldn't like to term them as weaknesses). Brush up on what you know so that you can handle any question asked in those areas. If the interviewer asks something in IR and you aren't very good in that area, admit clearly and frankly but immediately follow up with what you are good at. Anyone will appreciate when someone (1) admits what they don't know rather than beating around the bush or outright bluffing, and (2) gives an alternative. In a way, you will be leading the interviewer in a direction where your strengths are, from areas where you aren't confident.
This attitude or way of looking can be practiced by freshers and experienced people alike—in any domain or field.
Regards,
TS