What I did was right...right?

I have this female friend, we're both 20. I kind of like her and asked her out, but she rejected me. Now I'm worried that I messed up our friendship. The thing is though, had I not asked her out, I would have always wondered if she and I could have been together (I rarely meet girls and even more rarely get along with them). And besides, how long can a male-female platonic relationship really last? It won't be long before she gets a serious boyfriend or something and doesn't want to hang out with me anymore...I guess in this situation I just couldn't have come out feeling good...advice/experience?