It might be a mistake to put too much emphasis on mental states. The "flow" will not be there every day, or even most days. If you expect the production of scholarship to be consistently ecstatic you will be disappointed. Also, the ideas will frequently flow, but the prose might never just come out easily. You'll have to accept that.
On the other hand, the experience can be mostly positive and productive, on most days, and over the long haul as well. The idea that writing should mostly be a painful process is also mistaken, and a dangerously self-fulfulling prophecy. It can lead to the avoidance of writing. Most procrastination in scholarly writing is due to those two factors:
(1) Not be able to achieve a flow. The writer has it backwards, usually, not realizing that the flow is inherent to the work process itself, not something that exists before you sit down to write.
(2) Avoidance of anxiety. Writing is associated with frightening negative thoughts.