A few things that might help you:
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Medical schools care very little where you go as an undergraduate. They are not prestige sensitive. They care mostly about your grades and your MCAT score (and your state of residence for state schools). That means you want to go to college where you think you have the best chances of being at the top of your game academically. For many students, that means NOT going to a high pressure, highly selective college - in fact, many students at those highly selective schools get weeded out in the first year. Had they been at a less high pressure college, they might have not been weeded out.
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The fact that your parents will be disappointed if you don’t get into Harvard is their problem. They can join the other 95% of parents of applicants who were not admitted and commiserate if they wish. You know better, however, than to beat up on yourself for something outside of your control and so irrelevant to your success. And they will get over their disappointment when they see how well you are doing where ever you eventually end up.
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Black-mailing your kids emotionally into good behavior should be clearly acknowledged as exactly that. When you cave to blackmail, you just leave yourself open for future blackmail. You can love, honor and respect your parents while at the same time acknowledging that they are imperfect and not always able to love you and treat you in the way that would be most helpful, respectful and kind. The ability to act with that understanding is part of what being an adult means.