Never give up. Stop doubting yourself, stop telling yourself that you must be the worst writer in the world because you're not (unless you use text speak to write an entire novel, then I suggest taking a while to learn basic English).
I've gotten this feeling a lot lately - like I should give up writing because no one will ever read it - I am too descriptive, I have plot holes and my characters are seriously lacking. However, I keep reminding myself, over and over again, that practice really does make perfect; so just keep writing.
Writing is something that everyone can get better at, just like I have to practice writing essays, and writing my novels. That is also where the invention of editing comes from - it is helpful in finding what you need to improve on and what you're already good at.
If you seriously find yourself wanting to give up - take a break, read a couple of books, wait for inspiration to hit and then return to your novel. Read the last chapter you wrote, and then keep writing. Fresh ideas will definitely help spur you on.
One minute you'll think "God, this novel sucks! Ugh! I'd better drop it," take a minute, and eventually you'll start thinking more like "Holy crap! What if-? and then he -? Or if maybe -?"
Just keep in mind that you'll always be able to improve- and don't give up the moment you hit a rough patch in the relationship with your novel. It's like those couples in the movies - you'll work it out somehow.
