Usually when I try to find a book Google will get me there. But I am stumped now. As with most of my searches this is one I read as a child.
I have no idea of the author, probable British considering the setting, and I would expect female because of what I remember of the writing. It is a children's book, about a girl growing up with a female companion (governess?). The girl has vivid dreams, where she visits with a boy her age and they play together. At some point she moves, to Cornwall?, where she meets that same boy, who recognises her as well. Apparently she'd been having out of body experiences.
I also seem to remember that her companion found her love again. She had not been allowed to marry him before, because he was an atheist and she a minister's daughter, but when they met again he was a minister himself, so he was now acceptable.
Sounds like "The Little White Horse" by Elizabeth Goudge. Maria Merryweather goes to live in Moonacre Manor in Devon,with her governess Miss Heliotrope who finds that Old Parson, the local viicar is her lost love, Louis de Fontenelle, and marries him. Maria meets a boy called Robin in London, and then again at Moonacre. Loveday Minette, Robins mother , marries Sir Benjamin Merryweather, Maria's guardian, and Maria eventually marries Robin, having first reconciled Monsieur Cocq de Noir and his brigands to the local people.
Published in the UK in 1946, and winner of the Library Association Canegie Medal for being "an outstanding childrens book"