YC sounds like a fantastic opportunity. I would pour-your-heart into the application and if you do get the offer - write a hand-written letter (no email) and FedEx it to your internship contact. In the letter, tell your internship liason exactly what you told us - explain your feelings for why you regret turning them down (tell them how you feel, don't use logic). people buy on emotion and rationalize with logic (even the most logical people). Carpe diem!...