Game 1 - Magnus 10 Years Old vs OpenDeal. What went wrong???

I've been beaten by a 10 year old Magnus and I thought Black was winning... I think one of my weakest playing points is failing to convert an advantage into a win. I thought 9. Nb4 was completely winning and after 14. Qd2, this position had to be good for Black. But then somehow White managed to bring his queen into my queenside field but even after Move 24, I thought Black was still winning. How do you convert a positional advantage into a win? I can never seem to grab the iniative. Really frustrating, I've played 1800s and knowing I had some sort of advantage but then I failed to grab the iniative and play for a win. 

 Can anyone tell me if I ever had a real advanatge and how I could've converted it into a win or played better? And how do you personally play for a win konwing you have a positional advantage?