I mentioned before about editing a manuscript. I have a lot to say about this. Here's some of it: if I didn't have someone edit my work I would start sentences with comma's and apparently am a run on sentence addict. Whatever. I believe everyone should have an editor. Even if it's just a copy editor. Writers need someone looking over our shoulders and pointing out what's wrong. Personally I don't want someone telling me how clever my work is or how fun I am to work with. I want someone who'll challenge me. Someone who'll point to every misplaced comma, misspelled word, and misused adjective. Leave the hand holding to your partner, lover, or pet.
Which leads me to my second point. Find someone who cares and is smarter than you. Whether they care about you, your reputation, or themselves (if they're the named editor.) I'm lucky enough to have a wonderful (and very smart) person acting as my editor. I know she cares because she threatened me with excommunication from her world if I ever tried to publish something without her looking at it first.
And possibly last, but probably not: If you don't have one, grow a thick skin. Personally, I think everything I write is brilliant. Not everyone agrees with me and they are not shy about sharing it. Having my stories (at the very least) copyedited gives me confidence. Because I know deep down I still get confused about plural, singular, singular possessive, first person plural possessive WHAT! and all that jazz. I'm not an editor. I tell stories and the very last thing in the world I want is for someone to put down my story because they can't understand what the heck I'm saying.
Which leads me to my final point: if you're a writer you don't always need a publisher but you'll always need an editor.