Hello, I'm 19 years of age, I have been with my girlfriend no for coming on 6 months, I would say we have a really good relationship and she means the world to me but I'm scared of losing her because when it comes to having sex, my penis usually lets me down... She's on my 2nd sexual partner, 1st was a one night stand 2 years earlier so its fair to say I was pretty unexperienced before I met her. We used to have really good sex, two, three times a day in the 1st few months, but now its changed, I find it a lot harder to get hard, even foreplay doesn't really do much for me now and I have no idea why? Every time it happens it turns into an argument because I know she thinks its her, but it really isn't I think she's beautiful, when I try explaining its nothing to do with her she gets upset and says she doesn't want to talk about it. The only time we seem to have good sex now is on the odd occasion when its totally unexpected... But when it comes to doing it just before we go to bed, I'm limp and no matter what either us do it doesn't work! I need to sort this fast because she's left for uni in the last weeks and I'm only seeing her once a week, I seen her this weekend and I couldn't get it up, this upset the both of us because I really want to have sex but I know its not going to happen! I have no idea what to do because I'm scared of losing her to some that can satisfy her, as I know she gets a lot of attention from lads... Anyone got any help for me please?!

Sounds like you are so uptight that your going to "lose it" that you are thinking more about that than just letting it happen naturally. Also sounds like your girlfriend is putting a lot of pressure on you to "perform" when she should be helping you through this.
You said
"The only time we seem to have good sex now is on the odd occasion when its totally unexpected.."

So you know that it is working.

A lot of it is in your head, it's like a rolling snowball the more your worry the bigger the problem gets..

It could had just been a flooke one night and you probably worried about it constantly..

You need to give yourself a break.. and ask your girl to be understanding.. and give you time.. tell her it's a mental thing..

If after you tell her that and she still acts mad then maybe this girl isn't right for you... she should be understanding