Be patient with yourself and give yourself time to slide into your story. Use whatever crutches you have: play atmospheric music, or songs, or watch a movie that gets you in the mood, or read a book or a script or a poem. Or look at art. If you have only 1 hour to write, or less, it might be tricky to switch gears and snap into your story, but as you practice, you’ll get better.
It’s like acting. At first it’ll take you days to get into character. Then it’ll be hours. Then minutes. Then seconds. That’s when you know you’re a pro—when you can snap in and out of your story at will, instantly.
I’m not there yet, so you’re not alone. Today it took me a good hour to get myself into T.U.B.E. (some days I really don’t want to go back into all that darkness), and I have finally found a movie that has the opening matching the scene exactly (a car is driving through the Soviet countryside, covered in snow, in the dusk, along the birches growing by the side of the road…).
So you see, do whatever works. Just never quit. Never EVER quit. You’re closer to producing your masterpiece than you think.