Does anypony know anything about fimfiction.net? I was contemplating posting my story there, for some professional reviewing, but I'd feel more confident if somepony who was familiar with it would recommend it to me. I looked at it myself and it kinda struck me as a place where all the PG13 and above stories seem to end up. It's a little intimidating.
I can tell you that it can be intimidating but that that shouldn't put you off. There's all sorts of stories there, like you say PG13 and up, but there's filters to keep the mature stuff out of sight.
Other than that? Well, the type of story you write will usually attract the type of people that enjoy them. So for your story I wouldn't worry too much about trolls and such. You can give me a shout if you get in trouble and I'll make sure something's done about it.
Fimfic offers a lot of really good reviewers. WRITE members are particularly honest about stories and have very specific guidelines about reviewing. Usually they'll give you an extensive review with positive and negative feedback/observations. You can look at reviews from each reviewer to see their style before you ask them to take a look at your story. If they seem too harsh or too mellow, there's other members willing to help.
I gave the Fluttershy chapter a read through, and I can tell it's an interesting story already. I noticed a couple of typos, but I think what drew my attention was how incredibly self-aware Fluttershy was of her mental process.
Phrases like "With this question in my mind" are usually reserved for more immediate/tense issues, not necessarily Flutters waking up in the morning and wondering how she feels today. Unless she has a weird feeling that she can't identify.
Also, on that same first section, while you keep the mellow/shy tone well, I doubt she would feel weird repeating herself too many times, given she has to do it every so often. Maybe she would feel... sheepish? Or if it's something that bothers her about herself, frustrated.
Which brings me to another part I noticed... There was a point where Fluttershy starts shouting (as in, excited) I'd argue that Flutters never shouts, but, when she gets in that type of mood, she speaks loud enough to be heard and with all traces of her shyness gone (before reverting.)
Anyway, just a couple of thoughts, I'll read the rest of the story later, but like I said, don't be afraid of fimfic.