Overall its not a bad story. I like the plot concept and the characters you are using, however the story seems to feel forced somehow. The pacing feels off and the character conversations are so/so.
HOWEVER, this does not make it a bad story. I would suggest finish writing what you want to and then going back and flesh it out somewhat when you have more time.
Keep it up writing fanfic is annoying at times I know ( I am on fanfiction.net and potterheadsanonmous.com same userid)but you have the bones for a tremendously good story in here.
:>