I'm very sorry you're having these troubles.

It sounds like a therapist would be of great help to you. And medication could also be beneficial, even if just for the short term.
You mention that grief and sadness is not depression, but it is often considered "situational depression" in that there is a known cause for the sadness. Prolonged sadness is depression.

You also stated you can't do this alone. You must do this alone. Your daughter needs to live her own life. Set her free. In the long run, she will appreciate it, and your relationship will be the better for it.

A little piece of advice: Your post was difficult to read as there are no breaks. Many will take one look at that wall of text, and not read it. A post that long needs paragraph breaks to make it readable.