Erectile disfunction at 16?! Watch
Your body has a mind of its own, sometimes it really doesn't want sexual activity (particularly if such a desire has recently been satisfied) but you can force it with persistence and stimulation (porn, sexual activity) - however this forcing will likely result in a drop in arousal hence a weakened erection. This is normal and is not a sign of erectile disfunction.
Further, sexual activity with distractions (having to be quiet, having to leave the house, stress, guilt or shame) can send mixed messages to your body, resulting in a drop in arousal hence a weakened erection. You mention feeling addicted to porn, this likely creates anxiety and stress during masturbation - sending a mixed message to your body. This is compounded again if you're worrying about erectile disfunction.
In short, my recommendation would be to reposition masturbation as a positive thing in your life. Take a break for a week or so. Only masturbate when your horny, or as a reward; never when you're bored.
Remove the artificial stimulation (I'm not wild about porn at the best of times and would absolutely not recommend it for anyone under 18). Also think about your wellbeing, try to get more active, join a club or volunteer.
All this said if you're worried or have other symptoms pop along to the doctor but nothing you've said here sounds like ED to me.