Apostrophe S

Why is it that I have so much trouble with apostrophes? I continually screw them up, in both directions. That is, I'll write a sentence that has both a plural and a possessive or contraction, and give an apostrophe in the former, but not the latter. E.g., "[T]o reiterate, the FRC has expressed no problem with teacher's [sic] leading their students in prayer--so long as its [sic] to Jesus." So it's not that I over- or under-use apostrophes, I just use them in the exact wrong cases. The same thing happens with "their" versus "there"--I flip them around constantly.

It just makes me feel like an idiot sometimes.