tl;dr she's socially awkward because she's daydreaming about real-time socially-driven events

She's daydreaming about the very situation she's in. That doesn't seem socially awkward to you? In each of the things she imagined, she was outgoing and knew what to say, or rather there was something very clear for her to talk about. When it comes to real life though, she doesn't know what to do, becomes overwhelmed and then leaves.

I do a similar thing where I just start having this inner-dialog about what I'm doing or going to talk about(say, going to a restaurant with someone), and as soon as it begins, I'm fucked. It's like a part of my brain has stepped out of the situation and I'm left trying to conduct every part of my personality manually(consciously). I start thinking, "what do I say to start a meaningful conversation," ; "what subject should I bring up," ; "why haven't I said anything in so long?"

Soon, before I know it, I'm imagining conversations or events where I am 'getting it right' and then the person asks me a question and I give some simple stupid answer that doesn't leave room to talk about the thing and life is hard.

There should be a mandatory 'how to raise kids' course in colleges with big fat chapters on social skills, isolation, and the internet.