My skills in calculus are also quite bad.
This is going to be a barrier. It may be the whole barrier. Basically, DSP is a branch of analysis -- it's a collection of mathematical techniques that you can use to easily solve a certain class of differential and difference equations. It may not look like that, but that's because for the differential equations that it makes easy, the first thing you do is abstract away the "differential equation" part, and you often don't need to go back to it.
So -- you can't wrap your head around differential equations until you've wrapped your head around calculus, and you can't wrap your head around DSP until you've wrapped your head around differential equations.
I suggest going back to some good base material (the textbook for your course, perhaps?) If you've got a textbook with problems, then start working them. If you're having trouble with the calculus involved -- go back and get sorted on that part of calculus, then get back to the DSP at hand.