I can't remember almost anything about this movie.
Film genre: Probably a thriller (or mystery? idk)
Year of release: I was really young at the time, so maybe around 2000 to 2009
Language: Definitely English (probably from USA)
Characters: So from what I can remember, the protagonist was a blonde girl, between 17-18 (she was a teen). She also had a male friend who helped her through the story. (kind of love interest?)
Plot: The girl has a twin sister, but they were separated (probably at birth, can't remember, but they didn't grow up together because she doesn't know anything about her) and now her twin is trying to reach her by talking to her. This sister is trapped in an asylum or a psychiatric hospital and the only thing she wants is to trade places with her sister (she wants the other one to be trapped instead). I think she starts to speak to her through some kind of "power"/bond between twins and teachs her how to "lucid dream" (I don't how to say it, but this is the way she wants to trade places with her). The other sister really wants to help her, so does what she asks, and I can't remember how she figures it out, but she does. She never trades places with her because she finds out and either the other twin stays trapped forever or actually dies. (can't remember...)
I'm not a native English speaker, so I'm sorry if there are some mistakes. I looked for this movie for so long, seriously. I just can't find it. Unfortunately this is all I know about it.