What to do when on a date with a girl?

I have been told to always talk to more then one girl, but I always get stuck just dating only one. Yet I guess there are times when I talk to that one girl where there are lapses in my confidence, but for the most part I try to stay confident. A few of the girls I told that I liked but they didn't like me the same way. Also a girl that I recently dated stopped even calling me and all that. I kind of found out she was talking to another guy, so this has me thinking what I could have done better to keep it going and eventually close the deal. She is the main one where there was a lapse in confidence a couple times and it came it when we would text and call each other (this was after we kind of slow down the communication and I found out she was talking to another guy). My thing is she said she enjoyed being around me and that I was fun, yet all of a sudden it is like whatever she doesn't really want to even talk to me. I should also mention that at one point went out with her best friend, which I didn't know until a little after we started going out. So I don't know if that was the problem, but I eventually told her which could have been too late. For some reason I am still trying to figure out how to get this girl. In my head I am trying to determine if I really like her or is the fact that I couldn't get her, or didn't get laid.