There's an old saying among mathematicians that no one ever understands calculus until they teach it. I'm living proof that this isn't strictly true, since I do think I understand calculus at some reasonable level and I've never taught it (though I have tutored folks in calculus once or twice).

Nonetheless, one really good way to learn any subject is to teach it, and calculus is no exception. I've forced myself to learn many subjects by committing to give presentations on the topic at a fixed time, and then preparing the relevant lecture materials. Usually when I come back to the subject a few years later I'm embarrassed by the naivete of my initial efforts, but nonetheless I do learn the material and most importantly get over the initial hump of complete ignorance.

So try signing up to teach or tutor calculus, and see if that helps.