Ok, my take on it is that you can’t go to Harvard for undergrad. That option is forever gone for you. It will never happen.
Since it will never happen, I’d give myself a quiet moment to grieve for the path not taken, and then embrace the path that I did choose, for better or for worse.
I think you all are picking on him/her because of the names associated with the choice, but if you swapped them out for two colleges a few tiers down, the sentiment would still be valid-the OP is hung up on what might have been.
Don’t get bogged down in this-it’s ephemera, and it’s gone-focus now on what can be.
Some part of you believed Dartmouth was the right choice-believe in that part and go make it great.