This may not work for you, but I honestly didn't understand calculus until I worked through Knuth's Concrete Mathematics. There's a portion of it detailing rules on summations, which were (I realized at the time) the discrete equivalent of integration (summation of functions over integers versus integration which is summation of continuous functions). With my (stronger) CS than math background, it just "clicked" for me. You could check out the book from your university's library and see if this material helps you. I can't put my finger on which specifically now (too many years later) but various calculus concepts just fell into place as I worked through those portions of the book.