In this suspense/thriller movie which was probably early 2000s, a blondish young woman sees a spiderlike creature crawling on the ceiling. She is later put in a mental institution, but later she is discharged because she lied that she no longer sees the creature. The movie ends with the woman alone in her bedroom, surrounded by high powered lights and lamps.
I haven't seen the movie myself, but with some help from Google, what you are describing sounds a lot like the alternative ending to the movie They
The movie starred Lauren Rogan, who is blonde.
Two alternate endings were shot but neither of them made it to the final cut, they include:
- After the incident in the subway the film's plot cuts to nine months later where Julia is shown hospitalized in a mental institution. Julia manages to convince a panel of psychiatrists including Dr. Booth that she has regained her sanity. She then sees one of the creatures climb through an air shaft in the ceiling but continues to deny their existence. She is finally released and proceeds to set up high powered lights all over her apartment room. The camera then pulls out of her bedroom as she is seen sitting on her bed. A door creaks open in her darkened hall and the film cuts to black. (This ending was shown to test audiences which was deleted and re-filmed after test audiences responded negatively to the ending, for some reason this ending is unavailable in any DVD.)
- Julia wakes up in the mental hospital and sees that all the people in her story − Dr. Booth, Sam, Billy, Terry - are patients in the mental hospital and her boyfriend Paul is a doctor working there. The doors to Julia's room then break open and one of "them" enters and seemingly attacks her until it is realized that it was all a delusion fabricated by Julia's mind and she had been suffering from Schizophrenia throughout the whole movie. (Some versions of the DVD and all Blu-ray versions have this ending available.)
And from IMDB, the alternative ending you remember was described as part of the work print:
The work print features this alternate ending which is not available on DVD anywhere. When Julia is knocked out in the subway it cuts to 9 months later in a mental hospital. Julia convinces a panel of psychiatrists, including Dr. Booth, that she is cured. She sees one of the monsters climb through an air shaft in the ceiling, but continues to claim that "They" don't exist. She is released and goes home only to set up high-powered lights all over her apartment. The camera pulls out of her bedroom as she sits on her bed. A door creaks open in her darkened hall and it cuts to black.
So it sounds like you saw the movie before it was released. And the ending you are remembering can't be found on any DVD's