What else is to come that’s better?
OP, you aren't being unreasonable, as you aren't unreasonable to feel what you feel.
It's wonderful you had a lovely time with your DD, but that stage was never going to last anyway. Having children is all about loss as well as new experiences - every time we say goodbye to one part of their journey with us, we welcome a new stage (and this is true of the bad stuff, as well as the lovely stuff!)
I'm a mum to three teens. It's actually really, really hard (for me, everyone differs). But it's also amazing:
- They are hilarious, even when they are being frustrating or wayward
- They are forming into people in their own right, separate to you but bearing aspects of your personality (as well as their other parent)
- They are great company, you can do adult things together after a while: going to events, concerts, places, running / sports together, movies that you both enjoy - depending on your interests
Like other posters, I wonder if what you are missing is a part of your own life / identity, rather than specifically this stage with DD? This sounds like a really happy time for you and if you are depressed now, you may be associating this stage with happiness, and this may be even more the case as you struggled with fertility.
Can you discuss it with your DH, friends, or GP as relevant? Perhaps there are changes you can make to your own life to get back a sense of joy and lessen the feelings of tiredness?
I promise - every stage with children is filled with both joy and challenges, even when they are seemingly grown up! I would have felt that teenagers wouldn't need as much effort and care when I had small children, I had no idea what teen years would involve.