Do you keep editing stuff you've just written before going on to the next scene?
Are you stuck on dialogue that seems to be going nowhere?
Not sure how to connect the B storyline to the main plot in the A storyline?
Here's my advice...
And just write.
It won't be great. It won't even be good. In fact, it will probably be garbage. But it will be done and you'll edit it brilliantly later.