Welcome dogs4me! It's a long time since my children left for university, but I certainly had ENS with one of them (not the others, strangely, although I missed them all). I recognise your description of how you feel, and it took me some time to realise what was wrong with me. When my father died a couple of years ago, I was interested to realise that mourning him felt exactly the same (makes sense, I suppose!).
I can assure you that these feelings go away. They fade, and it's a bit like when you have flu - you don't realise how down you were until it strikes you one day that you feel better. All I can recommend is that you have patience and try not to be too hard on yourself. YOU WILL GET BETTER. Things will not go back to how they were, but you will adapt, and who knows, you may find that there are advantages to whatever changes have taken place in your life. I do hope so.