I think it's great that she's so capable. I do think it's a mistake, however, to dismiss feelings (ever) by saying, "You have nothing to worry about." That just makes her think you don't know what is going on. Instead, I would say that's it is normal to be nervous in new situations.
It would be better to suggest to her that she has great coping skills, great abilities, and that these will win out. So, what's she worried about? Friends and a new school? Then remind her that she has made good friends and good choices in the past. Remind her that nervousness can make people more alert and more inclined to notice things - including possible solutions. Is she nervous about grades? Remind her that she has done well in the past, that no one has to get all A's ever (especially in 7th grade), and that no one grade ever was enough to make or break someone. Empower her to take on more responsibility to talk to her teachers, that they expect this as kids get older. They are, in general, there to help her. And if she should run across a jerk - well, okay. There are jerks in all walks of life. So she will learn how to navigate this, which will be a skill that will help her with difficult girlfriends, boyfriends, bosses, police officers, admissions counselors (fill in the blank).
I would remind her of past achievements in negotiating tough situations, and support her in digging deep and finding the resilience to stand up to whatever challenges come her way, next week and next year and in 10 years. One great expression: "How do you eat an elephant?" "One bite at a time." So she will, with every challenge, break it down into steps and tackle those steps one at a time. Another image is a cross-country road trip. Sometimes we make a wrong turn or miss an exit (opportunity). But we may find other discoveries/sights, and we learn not to miss that turn the next time. Maybe it takes an extra hour or an extra day, but eventually we get there.