I have the same feelings. It makes me SO sad that my DD will graduate from hs at 16. I feel like I'm loosing two years with my beloved girl. And there is no way she'd want to do a gap year - she wants to go to college at the same time all her friends do.
BUT, there is no way that she would fit in any way, shape, or form with her age appropriate grade or kids. I often remind my self of how selfish it would be to keep my DD in such an inappropriate environment for years just so I can fulfill my own selfish needs at her expense.
Also, totally in agreement with what deacongirl and HowlerKarma say about challenge. While some people might see my DD's very occasional frustration as a sign that she's misplaced in a course, I see it as a rare and blessed sign that she's finally going to actually learn something in school!