Wild guess!
OP, first off, re: that A in physics...it's good to have realistic goals.
I had like a 73% average in physics (calc-based) and the almighty curve and even greater relative stupidity of my classmates turned it into a B+. But after taking 3 semesters of physics, I have to say, you either get it or you don't. And if you don't get it (like myself, no illusions here), you just can't get an A. Generally, in your average physics class, 60% of the class will be completely clueless, 39% will be pretty lost, and 1% (they usually have thick glasses and bad skin) aces every exam without even coming to class.
And kill the curve.
And I feel you in that with some courses, it seems like no matter how much work you do, you can't surpass a certain barrier. For me it was also physics, and right now is enviro chem. I studied my @$$ off, but it's quite possible that I might only get a B on the midterm I wrote yesterday (not so bad overall, but kind of a problem because I have to maintain a 3.7 this year for one of my top choice Canadian schools). In retrospect, I don't think that it's that bad - it helps you realize that you have limits and keeps the ol' ego in check, so various failures in the future (which will undoubtedly happen) will be easier to handle and less of a shock.