18 credit hours too much?

It's often good to give yourself one "easy" cl to keep yourself from burning out (anything that isn't math, science, or engineering is, by definition, easy). I would keep the humanities. The opportunity to switch gears on occasion will be welcome when you start dreaming of circuits and magnetic fields. This is quite a load though, I would consider dropping the DifEq cl. At my school we took that cl after multivariable calculus, which I ume is the equivalent of Calc 3. Doubling up on the math is going to make your brain hurt, and DifEq is an important enough subject that you should give yourself time to learn it properly. You'll be glad you did, trust me. You're not in school to race through it as fast as possible, you're there to learn. Good luck whichever way you go.