How do you learn to plan lessons?

We have a GTP student working with us at the moment. She is reflective and caring. My Year 12 students get on with her well and she has a very positive way of interacting with the students.
However, she is thinking about leaving the profession. The reason for this is the time that it is taking for her to plan. It seems to be taking all her time and getting harder not easier as she progresses through the course.

However, I am finding describing how to plan very difficult. I just know what I want to do. How did I learn?

I can remember struggling to plan lessons I left to the night before during my NQT year, but I always came up with something. What happened in the transition between the struggle and now when I have a clear idea what I will do in lessons up to three weeks in advance?
Is it simply experience?

Aside: it would probably help if we had complete schemes of work available to plan from and the wikid scheme is not easy for experience teachers to follow, so I can understand why a student teacher would struggle.

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Location:Bristol