Just relax man, as I said. It doesn't matter how long you have been apart.
This lack of confidence that your feeling has and will be experienced by probably all modern men on this planet, and those with partners have experienced this at least once in their lives. However, confidence can be gained by thinking positively and being relaxed.
From the sounds of it, the girl you are talking about sounds nice.
I'm sure she has to been interested in you to go out with you, especially that it is a solo date, not a group date or anything. A very good sign, indeed.
What matters now, is that you have an opportunity, a door opened... to show to her that you like her, and want to take it further.
Just let it flow, and as time goes, it will come naturally.