“It is the only point of getting up in the morning: to paint… to make something even better than before, not to give up…”
“If I knew exactly what I was going to paint in the next minute why would I want to do that?”
— Lucian Freud, quoted in Breakfast with Freud by Geordie Greig, page 10 & page 213
Not knowing exactly what you’re going to do next isn’t the same as not having a good idea of what you’re going to do next. It leaves room for possibilities and change, for letting the painting and subject suggest things to you and responding rather than sticking rigidly to a path determined before you picked up the brush.
If you do have a specific thought about what you’re going to do but then the painting suggests something else, follow it to see where to leads. You can always go back to what you’re originally thought to do (writing a note in a sketchbook helps remembering what it was!).