How are you practicing your purl stitches?
After I got the knit stitch down and did a bunch of rows of garter stitch, I started another swatch to practice my purl stitches. Well, I did every row purl stitch...which of course comes out exactly like garter stitch. This may not be what you did, but your post sounded very familiar to what I was thinking when I did my purls..LOL. So....now to practice purl stitch, I've been doing stockinette stitch ( knit a row, purl a row ). Hope this helps.