Sorry to hear about your daughters struggles. I can only offer a few ideas that have helped me personally, when I go through existential depression. Maybe some can be adapted to help your daughter as well.
One, I talk to people who are good at the name-the-feeling and repeat-back-the-concern techniques. So something like: "you're really sad, you are wondering if X" or "It makes you feel anxious when you haven't found the answer to X". I don't really expect anyone to have an answer for questions I grapple with, but it's nice to have another person just join me in the thought process for a bit. Sometimes, it's better for me to talk to someone who isn't a deep thinker at all,someone who will say, "Well I never thought of that. I can see why that would bother you."
Two, I find it helpful to keep a record of my concerns, thought processes, and the inspirations/resolutions that I've come to. I hate the word "journaling", but same thing. If nothing else, I can at least look back at past existential struggles and see that I found peace eventually. Also, it's good to limit the amount of time I spend thinking about hard things. So I might write and think for an hour a day and then make myself go about my normal life for the rest of the time.
Three, I try to turn my existential concerns into a goal. So instead of "I'm freaking out because I don't know what I'm going to do with my life", I might say "My goal is to learn more about how I want to spend my time" or "My goal is to spend more time doing the things that are important to me." Then, I can think more about manageable, concrete tasks to accomplish, rather than huge, overwhelming questions. Sometimes, part of my brain sees this as trivial busy work, like making an outline before you write an essay. On the other hand, I know I will feel better if I can take control of the process and gain some traction.
