Oh, I’m sorry you had a poor experience/relationship with your GC. I especially don’t like that your GC laughed at you. But try to give benefit of the doubt that you may have misunderstood what he/she meant by that, perhaps they were trying to keep things upbeat—who knows, we weren’t there to see just how condescending or intimidating your GC was. But regardless, it is too bad that you two didn’t feel like a team working on this.
However, as pointed out numerous times, you ended up with a fabulous outcome! Acceptance and strong enough financial aid to make one of our country’s best universities affordable is fantastic.
Yes, backing out of an ED-affordable acceptance to chase a dream school does not seem like a legitimate option (both unethical and also extremely, extremely risky in terms of outcome). It sounds like you are grateful for Cornell, so that is good news.
HIndsight is 20/20 and college admissions is so unpredictable. I sympathize with you that now that you are into Cornell you have a case of the “what ifs”. But realize that the odds of HPY were so skinny and your GC probably was using their knowledge of your classmates, your school, your own record, etc to point you in the best direction. There are times when it makes sense to just “go for it” and chase your dream, but many many people face the same dilemma as you, even without a pushy GC. Many kids struggle with whether they should use their best shot (EA/ED) on their dream school, or take a step down for something more sure, since a bird in the hand is worth 2 in the bush. The bird in the hand is often the wiser choice, but may not feel as exciting. This is not an uncommon feeling, so you are in good company.
My advice: try to understand that the GC was probably looking out for your best interests (even if they had poor bedside manner). Their guidance may be the reason you get to go to an elite school that you clearly care about. (If you had EA-ed to Harvard and hadn’t gotten in, the chances of getting into Cornell or equivalent would have been significantly lower RD, so you may have ended up at your state school, which may or may not have made you happy). Try to be forgiving and generous in your view of how they guided you. But if you think they did anything truly reprehensible, if they crossed any lines and did anything unprofessional that you think would hurt future students, you might take a week or a month off of this topic and then consider offering the GC or your principal some feedback). But mostly, as others said, try to rejoice in your good fortune!