This is a good example of why the old saying "Strike while the iron is hot" holds true. You had the chance to have sex, but missed that window. But it's not as big of a deal as it feels. There are plenty of other women and plenty of other windows.

There are hundreds of different possible reasons why she isn't texting you back. Trying to figure out "Why?" is usually just mental-masturbation (not to mention emotionally exhausting). Yeah it's possible that she thought you were too needy. If you feel needy, you need to work on that before pursuing a relationship with a girl anyway. Or maybe she backwards rationalized the lack of sex and concluded she wasn't super interested in you? As insane as that sounds, it occasionally happens.
But judging by how well you did with her on the dates, I doubt you were acting all that needy, if at all. So I wouldn't be too hard on yourself. Many guys out there can't even get dates.

I agree with Odyn that it is definitely possible that she just got back with a boyfriend or is dating some other guy. Or maybe she broke her phone. Or maybe she is on vacation. I mean we could probably sit here all day and list possible reasons.... But it's pointless because she is the only one that knows the reason.

I know it really sucks when a girl you have been talking to for a while suddenly drops off the radar like that, with no explanation.
All you can really do now is perhaps send her an honest message. If it were me I'd wait a few days to make sure, and then text something like genuine like, "Just wanted to say I enjoyed my time I spent with you and was honestly starting to like you. I'm disappointed you decided not to pursue this further, but I hope whatever path your life takes you on brings you nothing but happiness. Take care." At that point I would know I gave her a nice last impression of me, I kept a positive attitude and didn't act like a bitter, angry bitch like many guys do in that situation.
One last thought: if she doesn't ever contact you again than you two weren't as compatible as your emotions lead you to believe. If you truly connect with a girl, and she likes you in a meaningful way, she won't mystery drop you like that. Your best bet is to learn from this and to not allow yourself to become so emotionally invested in a girl before you two are actually a couple. Believe me, I've also learned that lesson the hard way. Eventually you will meet a girl that is much more into you than this girl ever was and you will hardly even remember caring about this situation.
good luck

"Just wanted to say I enjoyed my time I spent with you and was honestly starting to like you. I'm disappointed you decided not to pursue this further, but I hope whatever path your life takes you on brings you nothing but happiness. Take care."

I'm going to reply again because your story is so eerily similar to what I just went through. The girl mad into me, texted first everyday, wished me good night and even showed me pics of her friends on her phone and talked about how crazy they were and told me I would see for myself soon. I too didn't close etc, Two weeks later there was another dude. Let us know what you find out...but my money is on someone else. Also I wouldn't send another text unless for some reason you need closure?

I slipped because I thought I had her on lock...

This sums my situation up perfectly. Live and learn, there is always another guy which is why you better fclose asap or she is going else where.