I'm 17 years old and I currently am having problems with my girlfriend because I simply cannot sustain an erecting for intercourse. When we first start to kiss and get very very hard and I get aroused all the time by my girlfriend so I know for a fact I'm attracted to my girlfriend. However when it comes to having sex I'm able to put it in a couple of times with the odd going soft before hand. I would be very erect and then go completely soft just as I was going to put it in. When I do put it in though it feels great. I then go on to making my girlfriend cum by going down on her however if I'm down on her for more than 30 secs my erection would go straight away and I try to put it back in my girlfriend for intercourse but it wouldn't penetrate. Is this erectile dysfunction? I'm very worried as I'm quite young and don't know what to do. My girlfriend was understanding when it first happened but now she's beginning to become impatient and I'm worried I'm going to lose her if I don't sort it out. I've read it could be the food I eat or how healthy I am but I'm not sure which it is. I love my girlfriend a ton and want to sort things out because sex life is crucial.