^^ well I must say I can completely indentify with that (although I have improved a lot since I first started writing), still I type very fast and make a lot of errors. Thank god I have found a beta reader who is going through my chapters before I post them on this (and other) sites. So it might be recommended to do the same. And the flaming critic... well it's always easier to take a dump on something then to create something from nothing, so don't be bother to much about it. And if the things he says are usefull well learn from them (but I figured from your reactions you are already doing that ^^)
And you'll get better the longer you write ^^, my first chapters that were corrected by my beta were... well almost completely red ^^, but I got a lot better. It helps to see what you do wrong rather then just to hear someone say your gramma sucks (if you catch my drift) And a beta can be a great help with that.
And if someone starts complaining about how you write your story (so not about the poorly constructed sentences or faulty gramma, but really where you are taking the story) >.> Don't listen to it, it's your story so you write it the way you like it. It can be helpfull if you consider someone's advice on somethings (like 'maybe you should stall a little in your stories, to keep the tension up' or something like that) but if you don't want to don't feel obliged to. At first I thought my chapters were to short so I desperately tried to write long chapters... which didn't work at all for me. So I now write pretty short chapters, but they fit my story more then the long ones.
well good luck with your story ^^, I bet it will be great ^^