Instead of giving you specific advice, I suggest finding reading apps that will read the text out loud to you. In my work with the blind, I have found that this really helps. This is because the rhythm of the electronic reader is only based on the actual punctuation. So, although my blind clients may miss spelling errors, they completely get syntax, rhythm, etc.
I don't mean the really bad electronic sounding ones. I mean the ones that are more realistic. I believe Apple has some good ones if I remember correctly.
Also, on Microsoft Word, you can click the "view" tab. Then, click "Learning Tools". You can do some fancy stuff to make it easier to edit your paper!
For the specifics of your question, everyone has a specific style to their writing. I don't know your style. So, I can't really say what you might be missing. I think that I often see people attempt to use far too many slang words and phrases in dialogue. This tends to date your writing, and it can sometimes feel forced. That is really the main thing. If the dialogue feels "forced". Another thing I notice a lot is that writers will often mistake "struggle" for "character development". Just because you throw some mean things at your character does not mean your character develops!
The absolute best thing is to have a person who is very critical read it. Yes, your soul might be crushed a little, but you will truly know something is great when they say it is! then, you will feel like a boss!
I hope this helps you! Good luck!