What Should I Do? (Advice Greatly Appreciated)

I feel so stupid. I've been pursuing girls that don't want me and I've ignored the ones that do. This is my problem.
When I was at prom about 2 weeks ago, there was this HB6 named Gabby. She isn't ugly, she's just a bit chubby. I used to talk to her a bit last year but I never pursued her because I cared too much what other people think.
I was on the dance floor by myself at prom just making my rounds when I spot Gabby next to me. I dance to the music then I get closer to her. I gently grabbed her arm and spun her around. Then I started grinding on her. I never did this before but I just had this feeling that told me to take initiative. After we grinded for a bit, she was like, "That was fun!" And I basically ignored her for the rest of the night. I thought that I was being cool and building attraction but now I see that she was already attracted. I should've number closed.
Now get this, I saw her at HalfApps about a week ago and I just ignored her, once again thinking that was the right thing to do. Why do I keep wasting my time on the girls that don't want me, but ignoring the girls that do?
She follows me on Instagram. Her buying temperature was through the roof at Prom but I think I still have a chance. Should I slide into her DM's? I have nothing to lose but I just want to know what you guys would think. I need some experience, and I think she's a good girl that I'd like to hang out with. Plus, she already felt my ****. Should I contact her on instagram?

I've noticed that contact in person is better than sliding into her DMs. Also, she's a 6, so I wouldn't go for that. That's just my standards, though, so if you want to go for it.

Just go for it. Like Anima said, in-person is better. Ask her out. Because if you don't ask her out, well... she's going to say she doesn't like you. In 2013 I knew a girl I was in the school play with, and I made the mistake of just telling her that I liked her. I should have asked her out months ago. But she told me she had some issue with another guy, etc... and she even devalued her self in what she said to me. And that's one of my worst memories.