OK, having practiced structural, and conquered the choice 'tween mech. and civil, I will relay what I encountered. Mech. is harder in school. Structural is harder after school in my opinion. Once you are licensed, and have to stamp your calcs, you make sure your calcs are safe and hopefully efficient. The pressure is tremendous. You don't want a room full of kindergartners and their parents killed because you blew it.
Of course you don't want the astonauts to die either.
When you become an engineer, you will eventually be asked to consider will your design be safe and for how long.
Whatever discipline you choose, peoples lives will eventually be one of your concerns. If you want to do structural, then make sure the civil engineering college you attend has structural professors that have practiced professionally, and are registered as structural engineers, in the field. Take any structural and soils course, and always take every lab. You also need to learn some surveying too, and a summer internship in surveying is invaluable.
Engineering is a broad field. Do good in your fist two years and keep your options open. Choose an area when you get there is my recommendation. You don't have to decide until you are a late sophmore.
Good Luck, and learn your math!