Right here's a few comments
1. You have posted on a general discussion thread on a computer forum asking for girl help. Expect abuse and silly comments. If you have ever seen a similar thread you would know this.
2. You lack confidence. From what you describe this girl likes you or she would have given up on you by now.
3. No advice will give you this confidence. You just have to bite the bullet and ask her. The worst she can do is say no and you will feel bad for a few days. When I was younger I was similar to you (I had a childhood of being obese but when I turned 17 I lost it and became trim. I am told I was very,very good looking but never felt it as all those years of abuse and bullying had taken it's toll. I had a summer job working in a call centre and there was this drop dead gorgeous girld who kept making eye contact with me, sitting near me etc. By the time I plucked up enough courage to ask her out after 5 months we went on an incredible first date only for me to discover that a week later she was moving 400 miles away. However, the last night before she moved she gave me a present involving a basque and suspenders

I just regret all those wasted months) so just go for it and ask her.
4. It will get easier over time and your self confidence will improve but if you don't start now you will end up marrying the first girl who takes you to bed which is not a good idea.
End of advice.