No idea of publication year, I read it in 2003. Title is something like “As Time Passes” or “Time Lost” but I honestly only know that it has something to do with time in the title. No idea on character names, one might be Isabella ?? Plot description: has something to do with a girl traveling back in time to different eras, she meets a guy at one point who she falls in love with, but then travels to another time where she is maybe held captive in a royal castle where they think she is their daughter or something to that extent. I honestly don’t remember how she time travels, maybe because of a book she finds, or some kind of power?? I believe the front cover is like a pale yellow with a girl facing away with a braid in her hair.
Could it be A Traveller in Time by Alison Uttley? The description here sounds somewhat similar to what you described: http://charlotteslibrary.blogspot.com/2009/03/timeslip-tuesday-traveller-in-time.html
This link has a cover image that looks more like what you described (published in 1997): https://www.goodreads.com/book/show/1422149.A_Traveller_in_Time
Alison Uttley: A Traveller in Time
could it be : A Tale of Time City by Diana Wynne Jones ?