I think it is hard when there's any changes in situation. I'm the youngest in my family and remember when my next older brother went off to college and I was the last of five left :-) It was hard for me, nevermind my mother; fortunately she already had a grandchild or three by then.
I am retired and we moved from our home of 20 years a couple summers ago from our huge rambler to a tiny townhouse (and I terminated therapy at the same time). It's very hard to "start over" but when I look back and try to get back some of the friends, family life, etc. it doesn't work so I'm forced to look forward. We babysit the grandkids but not as often as their uncles and uncle's girlfriend :-) since we're further away. It has become a good time to enjoy things with my husband a bit more; since he's retired too we do most stuff together and I try to think of an "adventure" every few days. Yesterday I went to the barber shop with him and got my hair cut there (the barber took too long cutting it and it cost $5 more than my regular stylist so I won't be going back) and we did 4-5 chores together like that and it was very companionable and a nice couple hours together. I'm taking university classes online
http://www.umuc.edu and that has my interest at the moment.
But working on what I personally want to do "next" is hard work! What's the saying, "they go out one-by-one but come back two-by-two"? :-) Think of things you've postponed while they were growing up or things you use to do before you had the kids and see if there's not some spark lurking you can blow into a little fire. How about planning a party SeptemberMorn? The son with grandchildren and our daughter-in-law have become the "party house" where everyone gathers. We parents are all further away and they're more "middle" and have more reason for get-togethers. Get into the habit of throwing family parties so when birthdays, cookouts, babysitting, Thanksgiving, Christmas, etc. happen people/family/friends look to you? I didn't have many parties in my big house (no dining room or much sitting room in the living room) and now we're in a smaller place (big outdoors on the water though and we bought a boat, a child/grandchild-magnet :-) so I don't have many now except in summer we have some cookouts now and then.
We had my three stepsons over Christmas Eve for the last 5+ years though, started that "tradition" so now have that going too. Figure out a tradition or two you want to start and how to start it? We moved so we accidentally ended up halfway to my husband's brother's house, the boys "favorite" uncle ("Unc" and now the other two are vying to be called that by their niece and nephew :-) so we get some stopovers for visits to their house; they do Thanksgiving and Christmas usually. Can you do "care packages" for the away kids? One of my stepsons is away on a long assignment and I'm sending him a postcard every day :-) I even figured out how to do photo-postcards so I can take pictures of the kids when we babysit or things that happen (big snows, Halloween, Christmas lights etc.) and send them (whole new hobby, looking for interesting things to photograph :-) It's very nice having everyone around, too quiet without, but there are some good things about having time when you don't have to worry/do things when you're not feeling like it, etc.