I don't know whether you knit English or Continental, (I wonder if it matters) but this is my take on your problem.
I knit English. I recently discovered that I've been purling 'the wrong way' for rather a long time. I was wrapping the yarn the wrong way, resulting in twisted stitches. Instead of wrapping from top to bottom (counterclockwise) I had been wrapping from bottom to top (clockwise). This made my purls insanely difficult to knit into. Pay close attention to the way you're wrapping the yarn, and take a look at Amy's short purl video - the one of a single stitch. That's what really got it into my head properly.
Just because I'm easily distracted doesn't mean I