You may think that you didn't succeed on a date because of something you did or said during the date. Most likely, technically speaking, that is true. However, the reality is often times you failed before you even showed up: you failed because you had no confidence going in and it showed.
Sources of lack of confidence
Why were you not confident? She agreed to go out with you, so what were you worried about? There are several common drivers of this pre-date lack of confidence. A few of them are, in no particular order:
1. Fear of failure
You are afraid to fail because you "really really really" like this girl, and it showed. This behavior manifested in you not being bold enough, aggressive enough, and sexual enough. The confidence you had in getting her to agree was lost and she now views you as just some other nice guy who doesn't have the edge she is looking for in a lover.
2. Fear she won't find you attractive
If you have this fear because you are online dating and you put up pictures that are highly misleading or outright lies, that is your problem. The fix there is easy, don't lie about your appearance. However, often times, you are simply lacking in confidence in your appearance for whatever reason (despite the fact she agreed to go out with you knowing what you look like). If this is you, get over it, because it will show.
3. Fear of the skeleton
Do you have a skeleton in your closet? A health condition? A bad "past"? Out of a job? Depending on what it is, you may be right to worry, but often times, it's something silly that really shouldn't matter. The best solution is to not bring up your skeletons in the early going. There is no reason to do so.
She agreed to meet you
The most important thing to remember when you start feeling a lack of confidence going into a date is that she agreed to go out with you. Unless you got her out on totally false pretenses, you have no reason to not approach a date full of confidence. We all have skeletons and issues in our past and issues that we will have to address with our partners at some stage, but it doesn't have to be on a first or second date, and it shouldn't impact your confidence going into a date!