Writing is hard. Editing your work afterward is harder.
I'm in the middle of editing Tales of a Dying Star: Siege of Praetar, and it's tough. A writer is blind to his own work, and it's difficult to read it as a critic. Taking some time off between writing and editing helps, but it's still not easy.
TheWriteLife has a really nifty self-editing checklist. Here it is, in all its brutal glory. It's helped me, so hopefully it'll help you.