You decided to indulge in the madness that is NaNoWriMo. You’ve done this before, and you’ll probably do it again. You question every time if it is worth doing. Remember every time you push yourself a little harder, and you learn a little more.
I know you feel like you are cheating because you are not writing something from scratch. You are writing your fifth draft of your novel. You are not editing. As a friend told you, editing is tinkering with sentence structure. You are rewriting over half the scenes and adding new material. You will probably not complete all of it by the end of November, but you will have made substantial headway.
I know you’re disappointed that you can’t work on your new idea. But you know what the problem with it is. You don’t have a full story. You have a character with a backstory. You don’t have a plot for it yet, and you came up with it too late to do the research required. Wait until you’re finished with this first novel and then go research the scrap of a plot idea you have. The full plot will present itself when it is ready to be written down.
So roll up your sleeves and dig in. Remember the advice you gave yourself last year. Points 1, 3, 4, and 5 are the ones to keep in mind, especially point 5. But you have won NaNoWriMo before. You can do it again. Keep your chin up, you eyes on the screen, and your fingers on the keyboard.