NaNoWriMo! Hip hip hooray!
Celebrate! You deserve it!
I know that it might be getting hard, but the advantage you have now is that you can look back and see what you've done, then use it to motivate yourself to keep writing.
When you feel yourself slowing, look at your word count. Know that you've done that all on your own, and if you can write that much in the first half of the month, there's no reason you can't at least do the same in the second half of the month, right?
You've accomplished so much already. You should be so proud of yourself! The point of NaNo is to show yourself that you can make time to write your story. And whether you've got 500 words, 5,000 words, 15,000 words or 45,000 words, you've done that!
Yes, keep writing. Keep at it. Know that whatever happens between now and December 1st, you have words on the page, and that's what's most important!