After some quick research, this is what I found:
P0519 - Idle Control System Malfunction (Fail-Safe)
Trouble Symptom - Engine keeps running at higher revolution than specified idling revolution
Sounds like that's exactly what you're experiencing. Basically, your ECU can't get your engine to idle at the proper RPM. This could indicate a problem with the idle air control valve or the neutral safety switch. My bet is on the second one since several people have experienced this exact same thing before and when they replaced their neutral safety switch, the problem was fixed. One guy's was even covered under his factory warranty. My advice is to take it to the dealership and I bet they'll be able to fix it for you if you tell them what I told you - or maybe they'll just know if you describe the symptoms and give them the code. Good luck!
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)