OP, I have a 15yo. It's hard sometimes to watch her becoming more independent, but overall, I see it as a positive thing. It means that I have done my job properly. I am immensely proud of the young person that she is becoming and I want her to have her own life, pursue her own dreams. It's the natural order of things, and what we strive to achieve as parents.
It sounds like you've done a great job as a parent and your kids are maturing exactly as they should. They will still need you in the future but it will be different from before. What you now need to do is spend some time reflecting on your own life and what dreams and ideas you want to pursue apart from being a parent. Do you work or volunteer? Do you have hobbies? Do you spend time with friends and wider family?
My mum essentially fell apart when I left home. She had lost all sense of purpose and meaning because she had invested so much of her identity in being a mum to my sister and me. It was awful for her and pretty shit for us too as we felt guilty about getting on with our lives. As a result, I've always planned ahead for this time.
I will miss dd desperately when she leaves in a few years time, but I have a plan for my own life which will help to fill the void. For me, that means a bit more freedom with regard to my career (roles which involve more travel, for instance); more time for voluntary work (I'm already starting to build this up more as I have more free time); more time to invest in hobbies and interests (watching webinars, reading, exercising etc, and I was planning to join a choir before lockdown but that has had to be put on hold) and more time for my friends (I have been making a conscious effort to meet up with people more, to stay in touch etc).
It might look very different for you, but the point is, you need to have a plan. I started gradually building towards my new future a couple of years ago, when dd naturally started wanting to spend more time with her friends etc. I reckon that I will have created a full, enriching and purposeful life by the time she leaves home so that I won't feel that desperate emptiness that my mum felt, though dd will always be a much valued part of my life.
I'm sure you've given a lot to your family over the years. Now it's time to invest in you. In the long run, both you and they will benefit from this. So get dreaming! 