The more aware you become that you won't, chances are that will continue. You have to "go in" thinking it doesn't matter if I cum or not. Take away the anxiety. Take away your own thinking of how well you need to "perform". Just go at it and have fun. I once had a b/f that I found incredibly hot, super hot if you will, and I had a hard time when I was with him, similar to what you describe. It was him that kept telling me not to worry, you are a great guy, you time will happen. Relax he said, and referred to himself as just another guy and never feel you have to measure up because you may think I'm hot, etc. He broke down my inhibitions one by one, never got frustrated with me, and before long we were cumming all over the place. Not cumming is usually in your own head, once it gets stuck there it's hard to break but not impossible. You'll be fine. Just take away the "I have to cum" mentality.