I'd like to add a P.S. to the letter:
P.S. I'm also sorry that you're discovering that friends who are starting to notice boys in a whole new way will totally leave you in the dust if you don't feel comfortable with flirting or dirty dancing (which you call grinding). That if you want to spend time with the girls you've known and loved since Kindergarten you'll have to also spend time with boys who are girl crazy and flirt and tease your girlfriends, making them think the boys really like them when they're just playing and having fun.
I'm sorry that you're sad when your friends choose to be with other girls who are into the things you don't like. But I'm also proud of you for standing your ground, and reaching out to some girls that you wouldn't otherwise have gotten to know. Being 14 kind of sucks, but it does get better, I promise.
Stay true to yourself. You might even start liking those silly boys, and you might enjoy the attention, instead of it making you uncomfortable. You might even make some new friends. As the years go by and your confidence and maturity grow, you will look back and think about the lessons you learned during difficult times.
I wish someone had written a letter like this to me when I was 14. I probably wouldn't have taken it seriously, anyway. I knew everything back then.