Infants will start to lose the ability to recognize sounds of other languages before they're a year old if they don't hear them all the time. Therefore, there are sounds we can't distinguish between in other languages until we put in the effort to learn them. It's not just you... it's everyone. I have the same problem with we and wij, ze and zij :).
like stephenbal said, I experienced the same sort of delay in perception. i powered through the first few sections in a couple days, and within a few days of frequent exposure (and intense focus during listenings) i could start to tell the difference. if im not paying close attention though (like with this one), i usually end up just putting [x]ij as my mind's default. hey, at least that way i probablt have more than a 50% chance of being right. [x]ij does seem to be more frequently used than [x]e.