How can I help my daughter with her severe depression and her obsessive form of thinking?

Your instinct to help your daughter, is natural.

How you proceed depends a lot on whether she recognizes she has a psychological and emotional problem, or if only you see this from observing her.

Also, her age matters a lot in what way would be most likely to succeed in addressing the problems you describe.

If you and your daughter have different opinions as to whether or not she has problems, and she is above the legal age of when you have authority over her life, then you can only suggest to her that therapy may benefit her.

If your daughter is in your legal control, then you can locate a counselor nearby, discuss your situation with that person, and depending on the outcome, you'd be within your parenting right to take your daughter to a counselor.

Starting therapy without willingness to do so, is risky.  The person may feel resentful enough to not participate.   And, sometimes the counselor is skillful enough to find a path to your daughter, or any patient's self-interest and engage them in therapy.

Family counseling, regardless of your daughter's age, is another way to bring your concerns to your daughter's attention, in a therapy environment in which a counselor would be able to help distinguish the seriousness of your daughter's psychological problem, from simply a difference in viewpoints between you and your daughter, in how to handle certain situations.

Also, if you believe your daughter is a threat to herself, then instead of this slower route, get in touch with the emergency psychological assessment service in your town, for a more immediate response.