WRX's are tricky SOBs when it comes to shifting gears. My ex was like the most intelligent creature on the face of the planet, yet couldn't get it right, stalled most of the time, my dad - a professional driver for the past 30 years - stalled it on a intersection one time, it took me a while to get the right feel of it too after coming from a 5 spd MT VW. But now it's like breathing. One time I was driving with a girl I took out on a date and she asked if it was an automatic because it (I) was shifting so smoothly. Later she explained she meant to say tiptronic but didn't know the word.
Anyhow, you'll get it with more practice. For now just watch for that right foot giving it too much gas before you let the clutch out.