I've found a CBT* technique useful for overcoming that sort of anxiety (it's called catastrophising). I write down the situation and my prediction in a spreadsheet. An example would be: Situation - at work, I emailed [girl]; she hasn't emailed back yet. Prediction - She is going to break up with me.
Then when you receive an email back, you write down the outcome in a third column, e.g. received email back - we are meeting up tonight.
Looking back over the spreadsheet, you can see how accurate your predictions have been. I expect they tend to be too negative.
*CBT (Cognitive Behavioural Therapy) has a strong evidence base.
ETA: I hope that explanation is clear - I'm in a bit of a rush right now! I really wanted to explain it though, as it's had an extremely positive impact on my anxiety levels.
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.