Sounds like anxiety problems, which is totally normal when your about to lay a new girl or a really hot one, or both. The slightest distraction can cause a man to loose an erection, such as having to put on a rubber, switching positions.. I’ve been in this situation before and I find that taking a couple of deep breaths beforehand seem to help, but most importantly, you MUST relax and not think about the deed too much. Thinking too much has caused me to loose too many nights of potential sex.
Cliff notes: Don’t think too much, and Relax!