I read this book in London in 1992. It was a used paperback. The writer was a woman.
The heroine is a (rather dislikable) single, English woman of a certain age; she considers herself “on the shelf” and not a success in life. She has a small world.
Then she decides to reinvent herself and starts a diary. She writes her diary entry for the day at the start of the day and then forces whatever she wrote to happen. She writes that she meets a man and that day she forces a quiet dude into becoming her suitor, etc.
It was a recent novel: probably the 80s.