Ladies, where am I going wrong?

I'm a n00b that was hoping you ladies could help, I'm at a loss

So my girl of 6.5 years and counting is, we'll say, conservative in dress to the point where it drives me a little crazy. I don't want her to dress like a hoe, but she's 25 and consistently dresses like a 40 year old country club member, but without the hideous colors. She dresses well. Very well in fact, but sometimes it's like....please girl, wear a skirt that goes above the tops of your knees, or show a little cleavage or something.

I've tried to be covert about it, and then a little more overt, and it usually doesn't end well. I started just being up front about it, but she said it didn't make her comfortable. So I took the approach of tell her how hot she looks with stuff like that on, don't press, she'll warm up to it. Still wasn't getting anywhere. I've pretty much given up, but then there are moments where I seem to have success (She wore a thong for almost an entire day about a week ago, usually it's only a 5 minute thing while she seduces me and off it comes.), but then nothing for months.

After much poking and prodding, I know (or think I know) the reason is her high school years were brutal. She has hair that if unchecked, forms a massive poof, was a nerd in school, and didn't have any luck with boys. So in her world, she's not attractive, and doesn't deserve to try to be.

Normally I'd take this and say, do the whole comfort/tell her she's hot/let her warm up to the idea...but as I said, it's been tried and failed. Any suggestions?

TL;DR: Girlfriend is insecure about her body, how do I make her feel sexy?