personal history
It seems that you lack some personal experience in regards to relationships, am I correct? I know some here are going to hate me but I would give you pick-up related advice: what you need is not an easy fix but more experience in relationships. The way to get there is not easy: try to interact with as many people/women as possible, consider dance classes, etc... Competence breeds confidence(Mystery).
I wish you the best with this relationship.
I started going out with a fantastic girl a couple of weeks ago. Everything is great, except that whenever I've sent her a text message or email requesting something and haven't received a response yet, I experience significant dysphoric anxiety, fearing that her response will be not just "no" but "no and I don't want to date you any more". This is due to brain chemistry or personal history, take your pick—either seems like a possible explanation to me. But there's certainly no evidence supporting the idea that this is likely to happen, nor is the anxiety helping me prevent it or helping me in any other way.
Does anyone have evidence-based advice, or pointers to same, on dealing with this kind of issue? It is the only splotch on what have otherwise been the best two weeks of my life.