Oh, this is hard. How is this possible?
It’s possible if you schedule a fat chunk of time for your writing BEFORE you sit down and write, and it’s what will make your writing that much better. Without the pressure of having to stop (and without the pressure of having to produce words) your writing will come easier to you.
How to make it happen if there is no fat chunk of time to do it?
Take one of vacation days, a sick day, a weekend day—whatever day you can tear out of the teeth of your life—and spend AN ENTIRE DAY writing. Without a hurry. Simply letting it come you.
Afterward, note how long it took you to get into the “zone.” To get relaxed enough to stop worrying and start writing.
Now, armed with this time (one hour? two hours? twenty minutes?), you can plan ahead and schedule shorter chunks and bits, knowing exactly how long it takes you to relax. Use tricks to get yourself there faster (I meditate every morning for 10 minutes), and add them to your routine.
Your writing will thank you for it.