Every writer, no matter how talented or successful has detractors.  That's normal.  Having someone reject your work doesn't mean that your work lacks merit.  It may mean that you simply haven't found the right reader (or editor) who will love your stuff.

It may be that very few writing days will be golden at times.  Many will be downright bleak, and the words will come slowly and painfully.  Write anyway if you can manage it.  In a week or two, those same words may sound better to you.  And if they don't, you can always change them. 

Sooner or later, we all come up against something in our lives that makes writing more difficult.  When you're in a rut or you don't have much time, do writing exercises designed to foster enthusiasm, but which can be used later.  Brainstorm characters' names, book titles, plots or conflicts.  Nothing in writing is wasted, not even the words you eventually end up scrapping.  All of it feeds your creativity.

There was a reason you started writing in the first place, and usually that reason is tied up with the feeling you get when you read.  Read something you know you'll love and try to decide what that writer does to make you love his or her stuff.  Read something you know you probably won't love, and look for the things that convinced an editor to buy the work.  Read just for fun, to rejuvenate your senses and remind yourself why you wanted to be a writer in the first place.  Read a reference book you might not otherwise have picked up and ask yourself how you might use that material in a book.

No matter what happens, no one can take away the ultimate joy a writer feels when he or she writes something which clicks within his or her own soul. 

Remind yourself that the writing world is littered with battered individuals who gave up.  It is only by hanging in there despite the odds and by constantly striving to improve their skills that those who have made it actually managed to succeed and publish.  So, when you're tempted to give it up, step back for awhile if you must...and then come back the next day.  On the other hand, if the stresses of trying to publish are actually making you and the ones you love miserable, there is no shame in admitting that a writer's life may not be right for you.

Don't be too hard on yourself.  There is no perfection in writing, only improvement.

It's important to set a goal for yourself every writing day, whether it be to write a chapter, a scene, a paragraph or just a sentence.  It's also important to  stick to that goal.  Even if meeting it seems impossible when you first sit down, this type of discipline can pay off and you'll often find yourself writing beyond your goal or harvesting new ideas that wouldn't have appeared if you had skipped your writing session that day.  At the very least, you'll feel a sense of accomplishment, something most writers need desperately.

Remind yourself that every writer is different.  Don't try to force yourself to follow another writer's rules.  Some writers outline heavily, some don't.  Some write every single day, some save weekends and evenings for their family and treat writing like a nine to five job.  Some write in the morning, some at night.  Some write the book straight through, others edit as they go along.  Any way that works for you is the right way.

Remember that it's supposed to be fulfilling.  Be kind to yourself and remember that just as there is no such thing as the perfect book, there is no such thing as the perfect writer.  We all slip and fall, we all wish we could be better than we are.  The key is to enjoy the process, so....
enjoy!