There's nothing wrong with her! But there is something wrong with you; you don't have confidence and don't believe in yourself. You need to love yourself before thinking anyone else will. Be comfortable in your own skin. But if she likes you then go for it. Don't let your low self esteem get in the way of something that has the possibility of being great.
Well if you describe yourself like that then that might be your problem. Being a downer seems to repel people. I used to be really negative too and it took me a long time to change my attitude. For me it was a symptom of depression and it's something you might want to look into. At the very least it sounds like you have low self esteem. And when you have low self esteem and especially depression your mind kind of plays tricks on you in that it discards information that contradicts your negative beliefs about yourself. And a girl thinking that you're cute is powerful evidence, no?