Good morning again! It can be so hard to see someone we love hurting. The process of a child leaving can be really traumatic and tough even though we objectively can recognize it to be a time for great joy and celebration. The fact is that your daughter is going through a process of loss and grief and will need allowance and support to feel the great range of emotions associated with this time in her life. The way you can support her best is to just ride the emotions out with her with caution about criticizing the way she feels (even if you deem it to be somewhat selfish). Contrarily, encourage her to openly express how this loss has affected her and validate that you understand these feelings. Allowing her a safe place where she can express her sadness, worry, etc. will promote healing on her end.
If you want to be more proactive, keep her busy or try to lighten her immediate stress. Spend time doing things, consider a mother-daughter weekend away, get a gift for her that will lighten her work like a housecleaning session or month of meal delivery, convince her to take a couple of days off just to relax, etc. Distraction can be so helpful when your mind is consumed by loss so help her be creative in this realm as right now, she probably isn't thinking much about fun things she can still do for herself. However, rest and time to decompress is euqaly valuable.
Lastly, if you notice that your daughter is suffering from signs and symptoms of depression, such as change in sleep and eating habits, irritability, feelings of hopeless, lack of energy, decreased motivation, isolation/lack of interest in things, feeling constantly down/blue, then encourage her to talk to her doctor and also explore counseling. Big life events can be a trigger for depression even if it has never been an issue before and especially if it has. Depressed people may not realize it is depression and therefore may mot seek help. So if in the next couple of weeks you notice no improvement, then encouarge her talk to her PCP about an evaluation and possible medication, and then try to get her to look here for some counseling support.... https://therapists.psychologytoday.com/?utm_source=PT_Psych_Today&utm_medium=House_Link&utm_campaign=PT_TopNavF_Therapist
I hope this helps!