Difficult to gain and maintain an without physical stimulation?

Well Im 15 years old, I find it difficult to obtain an but once i have it its as strong as ever. But it goes away within 20 to 25 seconds if i stop stimulating it. I can keep an up through just fine. But i need physical stimulation to obtain an erection and even then it takes kind of alot, why is this? I do smoke but i ve recently cut down to 2 or 3 a day and im starting to run everyday. Jelquing seems to have improved my problem a little bit but im still having problems, any ideas?