A little while ago, a Facebook aspiring writer asked me for advice on how to keep the motivation going to complete her first book. I don't pretend to be an expert by any means, but this was my answer with some edits for clarity:
Congratulations on writing your first novel! It's an exciting, frustrating time for you.
I think writing your first book is more difficult in some ways than writing your next books. For a lot of authors writing their first book, they're sort of out there in the wilderness without any support and just winging it, hoping it'll turn out. One of the best things you can do is to join a writing group - they help to keep you motivated and moving forward. It's inspiring to hear how others in your writing group succeed and how they did it. Also gives you more ideas on what you can do. Having a critique partner is also motivating and encouraging.
Also, for writing, some days your muse is on vacation. In those cases, don't stop writing - just write about something else other than your book, update your blog if you have one, journal. Just something to keep you writing and your brain functioning. But don't stop writing.
You can try to set aside a period of time each day, a date with your manuscript every day, and just sit in front of your computer for twenty minutes. Often, just forcing yourself to sit there and stare at the thing helps to get everything going.
Be patient with yourself. Some people take years to write a book. Kelley Armstrong took seven years to write her first book and now she's a New York Times Bestselling author.
If you've run into a road block and don't know what to do, I have found that there is normally something wrong with my plot. It may take me a month of whacking my head against the wall before I realize it. So if you find you can't move forward, revisit your plot to see if something is not working there.
My advice, and some may say differently, is not to worry about finding a publisher until you've at least completed your first draft. Do take a look at which publishers you think will be interested in your writing and their guidelines ("The Writer's Market" is a must-have book for all the publishers, editors and agents out there) but don't approach them until you've polished your manuscript. For first time writers, the publishers want to know that you can complete a book you've started.
When it comes to submitting your manuscript, expect rejection. You will be rejected, maybe less than other writers, maybe more than other writers. I'm not trying to discourage you. I have a binder full of rejections from publishers and agents. Most writers have a drawer, file folders or boxes filled with rejection letters. They say (whoever they are) that on average if you're going to be published it takes five years. I don't know where this is from but in my experience it's fairly close to the truth. Most writers I know have 5-6 manuscripts collecting dust under their bed before they sell their first book. Some will sell their first book - I did but that's a little unusual - but it took me one year of writing, one year of editing and two years of schlepping it around before a publisher contracted me.
Editors, publishers and agents are not infallible. If they get your manuscript when they're having a bad day, then your manuscript could be rejected. After all, they are human too.
I hired an editor to do a manuscript evaluation on my book before sending it out to anyone. I wanted to know if what I wrote was total garbage - it's easy to lose perspective on your own writing. I found my hired editor a little pricey but absolutely worth the cost - I learned a lot from her including the fact that in her opinion the book was marketable as long as I made the changes that she suggested. And she was right, I made the changes and a publisher contracted it.
If you feel the editor you hired is totally off-base, then go with your gut feeling. Give your most honest friend your book to read, so that he/she can give you really, really honest feedback. You don't want to be squeamish about this, but at the same time you need to trust the person giving you the feedback and take it with a grain of salt. If you feel strongly that something you've done is right, then go with your feeling.
I hope I have provided you with some ideas.