Classic buyers remorse. No one should make fun of this poor kid for feeling something that millions of others have felt in the past.
Go to Dartmouth with an open mind. I know someone who turned down an actual acceptance to Harvard for Dartmouth, and I’m sure she questioned herself at first, but she ended up absolutely loving her time in Hanover and does not regret it at all. She’s not a drinker or into Greek life, but she loved it all the same. You may well love it too.