First, get this into your head. First drafts all kind of suck. Especially in the beginning of your career. Stephen King's first drafts suck less, but that's because he's been THE Stephen King for 30 years. You ain't there yet.
I have a novel, and it needs a rewrite, too, with major plot updates. One thing that helps me is this: Start with the old work in front of you, printed out. Create a new word file, and retype the old work into it. This way, your fingers are moving and the ideas are flowing. Then, when you get to a bit where the new plot differs from the old, Type the new plot where the old one was. If in the old version Jim turns left, and now, he turns right, make that change. Type out that whole difference, while keeping an eye on the old work. Then, pick up re-typing description from the old as needed, etc.
Some things will jump out immediately - when you retype, you are rethinking each word on a different level, and instinct will make you do a lot of corrections. Your subconscious has made some improvements, but you weren't taking notes at the time - this is now the consultation with the Subconscious You. But you're working the OLD version, right? You knew it had some good, but also had some suck, so it's not so crushing when you see suck that you *knew* about. Also, you're not second-guessing EVERYTHING, you're just rewriting, with your old work and your subconscious doing the dictation. You're just doing the typing. No stress.
When you finish your new draft, there are lots more things you can do to polish.
Start by reading this: http://www.holtuncensored.com/hu/the-ten-mistakes/
Then, read this: http://io9.com/5520058/4-danger-signs-t ... your-novel
Then, follow some of the directions in the second article. Do that search for " -ly " in your document, and gauge the level of adverb-usage suckitude. Congratulations! You've learned something about how you write, and that will be in your head as you generate the next MS. Gradually, those habits will reduce themselves.
Then, search for "were" "was" "had been" "have been" and fine-tooth-comb those bad boys. They're passive, or telling, or indicative of an unplanned tense shift. When you're analyzing a word choice for just one sentence, it becomes less emotionally wrecking than considering the MS as a whole. Many instances will be dialogue or innocuous, but some will be cramping your style.
Have 2 actors or friends (or you doing 2 voices) read your dialogue out loud, preferably on tape. Does it sound stilted? Unnatural? Well, then you will HEAR what to change! How great! When actors stumble over a line, chances are the readers will stumble over the line while reading it. There really is no better way to show off exact locations of problems within sentences, especially dialogue.
Ask your readers to identify crutch words. I bet you can find a few of your own. Search your document for them - surprised at how many times "ecstasy" popped up? Well, time for a good thesaurus, eh? And the search-and-replace function is your friend here, identifying problems which might need correction - not judging your writing! Just fixing issues.
Once you've handled these mechanics (notice, I did not call this "writing" ), you'd be surprised at how much your telling got turned into showing. The best part is that you're working with your writing the same way a kid dismantles a toy to see how it works - your baby becomes useful, nuts and bolts knowledge that will never leave you. You've seen inside the belly of your work, and you know why it's not ticking - time to oil up the joints and clean the cogs. You get better every time!
Last, because it bears repeating - life is a succession of failures. You either learn from them and fail differently next time, or you don't move forward. We all fail at relationships, and do better the next time. We suck with money and eventually learn how to bounce fewer checks. That's why older folks know so much - they've tried so many things and failed at them! They learned the hard way, and now it's your turn.
Rejoice every time you fail to write a sentence well. It's an opportunity to learn. But you can't do that if you don't write, because that's where that not-moving-forward ickiness comes into play.
Don't worry about your writing! That's the fun part! Once you've written, worry about your mechanics. Fortunately, you can do that in easily manageable maintenance chunks, and you can learn to separate because you're not chopping up your baby - you're making it smarter, sexier, and more likeable with a few fine-tuning tools.
That's not so bad, is it?