I couldn't wait for my first mammo! I was ready to get a clean bill of health and move on. I think all of us are a little anxious having that first one.
In my experience, a 'diagnostic mammogram' was a regular mammo with two different radiologists looking at it. That's the definition of it where I live. I will have a diagnostic mammo from here on. Not sure if that's what you will have. I'll be having my 6th one in a few months. I'm usually OK till a day or two before, then the bad thoughts creep in. I usually brush them off pretty easily.
Staying busy is the best thing you can do. Like Mary, I try really hard to focus on someone who is having serious problems, pray for them, and thank God it isn't me. Worrying about the results of any test won't change the results of the test. I wish there was a 'worry switch', and we could just turn it off. I do think we have some control over it, because I was a real worrier, and now I'm not.