I know firsthand that having a weak foundation, one will eventually hit a brick wall trying to learn a subject that presumes that one is comfortable working with that foundation, even under examination room conditions and emotional stress.
It is one thing to idly watch an instructor put an equation on the chalk board and mutter, "Yeah, that makes sense".
It is another thing to walk up to a blank board and put that same equation on the board unassisted.
And yet another thing to know when to apply that equation (or some other equation) to a real world problem.
Working lots of problems is key. Do MORE than is asked of you.
I have never been assigned enough problems to really get good at something. It always took me more.
Learn it like your career depends upon it.
Someone's life might even depend upon your mastery of these subjects!
Calculus is not difficult provided that one has a solid foundation in the algebra and trigonometry that it is built upon.
It will only take you a few months to master the foundation first OR you can skip this step and flounder in calculus for several frustrating months and ultimately fail horribly.