Anna
Wow, I am not sure that I can really add much to Terry's answer. It is quite thorough and contains a great deal of great information.
As a parent, however, I agree with you. Your daughter must find a way to get her depression and ADHD under some type of control. I have told my son many times, first, it is important to learn to control your (ADHD and Bipolar in his case) instead of letting it control you. Once you have accomplished that (and that is a LARGE accomplishment) then you will be more able to overcome the obstacles that life sends your way.
This is very difficult. Please also keep in mind that children and adolescents with ADHD are often immature for their age, sometimes by several years. So although your daughter may be a certain age chronologically, she may not be emotionally. She is therefore living expecting to be able to do the things that other people her chronological age can do. This may not be possible for her yet, but with time and work, it is very possible.
Taking time to get her to this point is a good move, but, unfortunately, as our children grow up, we don't always have the final say, they reach an age where they are able to do what they want (but must be financially able to do so).
It is a hard and difficult decision. Let your daughter know that you love her and want what is best for her. Set some goals for how she needs to do at a local college and what expectations you would like to see before financially making an obligation to have her attend college away from home again.
I hope this helps.
Eileen