Self-study likely won't cut it unless you are a prodigy or something. If you want to be an engineer, you have to be decent at calculus, especially Calc II. If you want to be a good engineer, you have to be very good at calculus. If you want to be a great engineer, you have to own the material and have a pretty deep understanding. There are a ton of engineers out there, a decent number of good engineers, and only a handful of truly great engineers. Which do you want to strive to be?