Lets see...
First, another interesting chapter. The flashbacks took up much of the chapter but you segmented between and into them nicely so no real complaints there
Second, work a bit on your character development. Although it was easy to determine who "Mi" is based on her description in chapter 1 and her parents profession in chapter 2, you never really introduced her in or into the story. Also need to explain how/where Harry developed the nickname for her...
Third, grammar could use some help. It wasnt too bad and the story was more legible than others Ive seen but it could still use some improvement. I know you dont have a beta so just try to work on it... Maybe when you finish a chapter, walk away for a few hours before trying to proof it, if its fresh in your head your brain will automatically fill in the blanks of what you wrote with what you meant. If you take a break, when you return it will be more like reading it for the first time and thus easier to identify problems.
Fourth, dont use HP stories in a HP story... Its incredibly bad form, tacky and can be quite paradoxical, its like using a word in the definition of the word, you could potentially do it but you shouldnt. There is plenty of other literature out there to choose from. It doesnt even have to be good literature, just as long as its something different.
Fifth, dont take my constructive criticism to seriously, keep up the good work and post again soon...