Ok, I see #13 now.
The number 1 MOST helpful thing you must do, regardless of if you stay or leave, is to let go, right now, of the idea of a dream college. Dreams are not reality. Being ho hum at a prestigious boarding school is not going to help you get into a tippy top college. They will know your school and you will be compared to the kids in the top 10% or so of applicants. The unhappy kid with average grades is not going to be a reason to admit you, regardless of your boarding school name. And yes, they may indeed think you found it too challenging.
You can’t forever regret something you have no control over. You can’t make a college accept you. If you don’t get in, you will never be told why. You need a balanced list of colleges that you can get into based on YOUR stats, not based on the name of your boarding school.
Look at it this way: what will you regret more? Staying in a place that makes you miserable, or going home and being happy, and letting the college chips fall where they may. Go home.