In the first book, the protagonist is a young girl who travels between our world and a different one (possibly through a wooden gate in the barn next door that she finds the key to in an old coat?). She falls in love with someone in the other world, but at the end of the book, the world sinks underground in a storm (a la Atlantis) and she loses her love.
In the last book, she encounters a reincarnation of her former lover (they both have dreams of the former life). He's named Luke in this world, but when she realizes he has signed his soul over to the bad guy boss (characteristic nick on his wrist from a knife used to sign the deal in blood), she kills him by stabbing him in the stomach. Some time later, she makes a deal with the bad guy boss to drink a forgetting potion so he will stop bothering her. The forgetting potion was from a friend (named Kyle?) who is the son of an evil witch (Moganna? Morgan la fay?).
I probably read the book in the early 2000's.
I hope I'm not getting a couple series mixed up with each other... I did a lot of reading as a child. My inability to recall the name of these books has been bothering me for the last 4 years.