Its probably purely psychological. You are worried it will happen, so you get anxious and it starts to happen, then you PANIC and go completely soft. You need to break the cycle and slowly as you get your confidence back, you can break the cycle.
Two major ways to break the cycle is;
1) Don’t touch your dick for about 2 weeks, no porn, no sex at all! You will be so horny, it MAY get you past THINKING about it when you get oral.
2) Cialis or viagra. This will help quite a bit breaking the cycle.
3) Combination of both. This is probably the best shot at it. No sex for two weeks AND Cialis or Viagra.
Really, once you get your confidence back, and quit being ANXIOUS during oral sex, you will give your body a chance to react normally. Also, learn to just tune into the sensations and get out of your head, your thoughts about it. I like to bring a hand held mirror and watch from all different angles when getting head. It helps to get you out of thinking about it, by giving you new and exciting angles to watch from.
The biggest thing here is don’t take it seriously, consider it just temporary and that you will be fine. The more panicked you are by this, the worse you make it.
The other thing that works pretty good is to do a 69 position when getting head, that is a great way to be distracted, by something very exciting. The moment you concentrate on giving her pleasure, your dick will get hard as rock.
You will be fine as soon as you stop taking this seriously.