A very heart wrenching situation, I'm sure. That poor girl. I do like the idea of your story, but there is a lot of work to be done. Can I make a suggestion? Run it through a spell check, run it through a grammer check and then read it backwards, sentence by sentence. That way you can catch individual grammar errors and such.
When you're finished doing that, I suggest reading it backwards, paragraph by paragraph to check the coherency and flow within each paragraph. This is a trick I learned through my many writing teachers. If you read it normally, your mind will fix any mistakes (such as to, too and two), and you will mentally add in the punctuation because you know how it's supposed to sounds like.
Your weekness seems to be run-on sentences. I've found some websites to look at that may help you.
http://ace.acadiau.ca/english/grammar/runon.htm
http://owl.english.purdue.edu/handou.../g_sentpr.html
http://cctc.commnet.edu/grammar/
The last site is one I used frequently during highschool. In the end I purchased a grammar book (which is good to have as well).
You're also welcome to ask me for help if these sites don't help you at all or if you don't understand.
I also found the story kind of fast. Maybe draw it out; let us meet the characters. I think an intersting approach (even if it's just for excercise) is to tell the story from the perspective of the cat.
On the whole though, the story has potential and I would love to see it again, perhaps when it's all fixed up and you've had more time to work on it! Wonderful job.