D.K.
This is the hardest, and it is shared universally by mothers. You put your heart and soul into raising them, and it is lots of work when they are young, and then they grow into your best friends and define your family, and suddenly they are off and you wonder what you were doing before they came on the scene, and somehow it holds little interest for you, now that you've been a mother.
For myself, I tired to "get a life". Not that I didn't have one, but I tried to come up with some things that greatly interested me that were not centered on my kids. I remember my own mother went back to college for a few classes, and joined a floral arranging guild. I put more effort into my small business I started, and I began to ask friends to lunch--something I'd always done with my daughters instead.
My own mother would cancel a friend's lunch if I came over on the spur of the moment. She said she preferred my company to any friend. Which was sweet, but it made me realize that the love of a mother towards her child exceeds the love of a daughter to her mother...mostly because the daughter just has not yet experienced that strong mother emotion and bond yet. And by the time she does, it will be for her own daughter. So basically, the mother is the one who is eventually going to feel pain at separation. Husbands/fathers seem to be less connected and feel less pain about their daughters growing up and moving on. They expected it. We mothers are sort of surprised by it.
I realized that mothers with families still around them are the "Queens" of this planet. Sought out and loved by their family and dedicated to loving their familiy. Heart of the home. Once their daughters grow up and move on, then the daughters become the Queens, starting their own families, and we move out on the circle a step away from center, and become less vital. It is hard to face, but it is so. Their husband and their children become pre-eminent. Even our advice is not sought often...seems outdated to them, even though we just went through raising them!
Staying busy is key. Find a hobby, read, join a club, garden, find friends, etc. No, none of it seems as desirable or important as the job (mothering) you were doing, and it is not, but you've just been retired from that job, and you have to re-invent your life and stay happy. I found I finally had time to give my husband some attention, do some of the things that I'd put off while raising kids, scrapbooks, family history, exercise, etc. Not that any of it was as great as being a mom, but it does take up your time. If you find a good cause that you can support, now is a good time to do it.
You can still be a part of your daughters' lives, but it is going to be on a far less frequent basis. I have one daughter who comes and visits me once a week and occasionally I think it is a chore for her that she has assigned herself in order to be a good daughter. The other daughter calls or connects more frequently, but she doesn't want advice and doesn't want to talk about my challenges. Wants it light and breezy. It just isn't the same after they move out....empty nest isn't easy for anyone.
I sure empathize with you. It does take a huge adjustment and it is challenging to go through. —D.K.