the point of NaNo is to have support and to get yourself writing. If that means working on another story - just start your word count from wherre you are.
If that means you only write 150 words a day, then that's cool too. reaching 50K is a perk, but not really all that important.
I mean really, people who reach 50K get no more than bragging rights. Which, well, isn't really worth stressing about.
n Enjoying yourself and trying is far more important, in my mind, than actually getting to 50K. Write what you want. Ditch the story you don't like and write when you can.
And if you feel like it is too much, then stop.
There isn't anyone to report to but you.
I feel the same way about "having" to do something that you do - and I bargained with myself that if this became too much and felt like having to do homework, that I wold stop. No point in making myself cranky for something that is suppoosed to be fun.