Why She Doesn't Reply Me?

I am a very common girl doing a very common work, sales. It's not so easy for everyone I think. Put my full heart in it, but gain little from it.
Just like people always says, maybe it is hard at the frist, and it would be fine in the end. I usually imagine that one day i would be so great and so confident in doing my job. But, it needs my pain, my hard work, my endless effort.
I know everyone has the same trouble with me...
Today, my customer doesn't reply me again with the problem about loading time related questions. I feel so terrible to myself--why I would bother her again and again. Maybe she just feel I am a crazy girl and I am so boring. What can I do? It is urgent to know those details in a degree. I failed to contact her because I am so shame to call her, but just e-mail her or text her.
Things would be left to tomorrow again. Why I am so inconfident? Why I am so afraid of losing this customer? I am a loser, am I? Coward...