3 weeks chat = too long, need to strike when iron is hot, before interest peaks - get her out.
1 date and bye bye after = obviously your online persona > your real life, could be a dozens of reason but we know she lost interests after seeing you. So if you're not an attractive person in terms of looks, then you have to make it up with an attractive personality. If you can put some effort into your own grooming, fitness etc and make yourself attractive overall, that's for the best and also good for your own confidence building.
Learn to be funny, learn to be fun, learn to be light-hearted, stop being so clingy and whiny, I read your 1st post also feel that you are too much into it. 1 date shouldn't warrant this much emotions lah.
-Source, I dated >50 women, bedded >half of them, married the most perfect of them all and happily ever after