OP, I get you.
My youngest (therefore my baby really) left first at age not quite 19. It was quite a sudden thing as an opportunity arose which I didn't have time to digest, before she was off. It hit me hard, as for one she and I are very close, but also she had a rough few years prior, and basically I was worried how she would cope. And I am a single parent.
I feel like I cried for at least 2 months solid. We could always speak just whenever as she was in a different time zone and far away, so some days I didn't get to speak to her at all, which made things so much harder.
Just as an aside, it was during the year that the Dr decided that I was actually having enough symptoms in peri to warrant going onto hrt, so I do think that helped somewhat.
I think you do just get used to your new routine, that is what I found so very difficult, and my whole life revolved around her, school runs, medical appointments special meals/ etc etc. I really did feel like I didn't know what to do with myself some days, as our days were usually just so full on busy.
I didn't really have a proper heads up, I thought my eldest would leave first to be honest, so the sudden change didn't help. As I thought about it, I think if I had prepared a bit better, I would have looked into starting some new hobby, getting my head around starting to go to meet up groups etc. Just changing up from the normal day to day routine.
Even now almost a year in, I still find I need to have some 'goal" to work towards, otherwise I start to get anxious. So i re-did some things in the house so it was all nice for her when she came home for a break, Christmas was hard as she couldn't make it home, so I just plunged myself into having everyone else over, doing all the food, just generally keeping busy.
One of the things that annoyed me was that some people make fun of it. Oh they say, sniggering, empty nest syndrome, are you broody?.... I don't know why some people link empty nest with being broody, it is not that feeling at all (well not for me).
Can I just say now, that she is flourishing so much more that I could imagine. She is enjoying life and absorbing everything that she can. She would not have been able to do this at home in our small town. We speak when we can, and I am always the first to know if she is upset, down, stressed.... as she knows I am always here, perhaps only on the end of the phone. But she knows her safe space is here if she needs it. I know I have done the best I could for her, to grow into a strong, brave adult.
Wow this turned out long. I still need an outlet from time to time as well. A year isn't all that long ago really.
So I really do get it OP.