Learn from concrete and constructive feedback, ignore vague criticisms and inarticulate judgments. If someone thinks they can judge you negatively without giving you specific feedback in this process, they do not care about you or they are thoughtless at the moment or both. This INCLUDES people on medical school admissions committees or residency program admissions committees. Try to brush off unconstructive, fuzzy, and especially negative criticisms for what they are: slips or gaffes or social misbehavior on the part of the criticizer, saying more about this person than about you. Get your boxes checked, make sure you know yourself, and why you're doing what you're doing, and always keep this, rather than what others say, at the forefront of your mind. This is not always easy, but it is necessary to do in order to be successful and also keep your peace of mind. Remember that although success and ambition may bring prestige and therefore the positive regard of many, it can also occasionally, in the process, provoke negativity from some--from any number of different quarters. Know your friends and what advice you should and should not listen to. This will help you to be and stay happy. To risk grandiloquence and to take possibly extreme examples, history is full of successful people who ignored snap judgments, even from those on high, and went on to be successful despite the occasional skeptics. In many cases, these skeptics had more specific and relevant concerns than concerns about "it." "It" is the very last thing you should pay attention to; pay attention to things you can control.
Hope I am not rambling too much. Long flights and lack of sleep make me a bit philosophical.