Hi! I got waitlisted for Exeter and Mercersburg this year- what can I improve?

You sound like a great student with a lot to offer a school!
From where I sit, the mistake was: you applied to three schools, not ten. Lots of people get 3 rejections or waitlists! The difference is that they applied to seven more so they might also be getting some acceptances in the overall mix.
You sound awesome though!!