I think some of it is hard-wired and developmental WRT to your DSD. I raised my DD, from birth, using Waldorf and Montessori principles. She was waterborn at home, BF for four years, fed organically. We hung around with only homeschoolers. She has a healthy weight now at age eight. AND she is obsessed with sugary treats, popular preteen stuff like High School Musical, the dreaded Hannah, hairstyles, clothes, and computer gaming. She acts "bratty" and unappreciative at times. She rolls her eyes and stomps around and thinks she's deprived (heh. not the case). She's also smart and independent (read anti-affection! bummer!) and hilarious and way into theatre. She's my bio kiddo so I do love her unconditionally, but sometimes she bugs the crap out of me. :-D My partner struggles to remain positive in her interactions with her. It's a rough age for some kids and their parents/steps.
Here's what, I guess, I'm saying. It's not all your fault. You don't deserve to be flamed. DSD probably is acting in an annoying way. She's also totally normal. So are your feelings! Lower the bar for yourself. Stay as positive as you can for her sake because she IS, after all, an innocent kid. Ride out this phase and remember that this too shall pass.
I remember when DD was a baby. I was so sure I'd never have one of "those" kids. Well, you know what? EVERYONE does, at some point. There will be phases your son goes through which disappoint and embarrass you, but they will pass. May your marriage last forever - but if, in theory, your son ever had a step-parent, that person would feel a lot like you feel during those phases. And it would be YOU saying the words you're hearing now from your DH.
Bottom line: you're totally okay in feeling how you feel. The fact that you feel so guilty says that you care and that you have high expectations for yourself. You're a GOOD stepmom. Cut yourself some slack and do the old "accept what you can't change, change what you can and do your best to know the difference." I find I connect best with kids when doing something I really like. For example, the park bores me to tears but I love riding bikes together and we find lots of chances to talk and laugh that way. Maybe you and DSD have some undiscovered hobby/activity in common? best of luck!
I am so with you on the buttcrack problem. Clothes these days are made too low in the rise for "style" purposes, without the consideration for how active children are compared to teens and adults. you might want to do a little shopping for higher-rise pants. My kiddo eats healthy food but also lots of junk/sugar compared to the average MDC kid and isn't chubby at all. I was the same way. your DSD may just be about to shoot up in height, y'know?