Even the best and the brightest can not understand everything put forth in an engineering curriculum. The ones who do best are usually those who try exceptionally hard on their own but are also willing to ask for help either from friends, or a teaching assistant, or the professor. And though you may be used to being a big fish in a little pond, there are still rewards for those little fish in the big pond of engineering. Even exceptionally good students often find that they may struggle in engineering. But do not worry about the occasional failure.
Industry often hires engineers who have less than even mediocre grades.
That is one characteristic of engineering programs. You do not have to be the top dog to be considered successful. Just completing the program is a success in itself. Coursework Engineering students usually have different types of coursework than humanities or social science student would. Engineering curriculums focus on problem sets, labs, and projects.