I enjoyed math in my school years and for the most part, excelled at it. Until I got to calculus. Probably the most powerful mathematical discipline, calculus eluded me the one high school course that covered it and my mandatory first year of university calculus. It wasn't until I had to re-take the compulsory course that it suddenly started making sense to me. After it clicked, it became fairly easy. But it doesn't always click since it is a conceptual math.
But the power is undeniable. My high school physics teacher once told the class that if we already knew calculus he could teach us the entire year's worth of physics in three weeks. I have no doubt he was right.
Calculus in 20 minutes, for those with some existing mathematical fundamentals: