Oh I can empathise as I am so sentimental it is unbelievable
. My youngest is now 7, and 3 years into primary school, yet I still get regular pangs of sadness that these baby/toddler/pre-school days are in the past, and time is getting faster and faster... (Not helped by the fact I am waiting for covid to pass before returning to full time work, and I currently work from home part time so the house being quiet isn't great lol). I miss the toddler classes, the walks in the park with the pram/buggy, nursery runs (I hate the school run lol), etc, but I think everyone feels like this in some form of another, just some more intensely than others.
I don't feel this sentimentality every day, but regularly, and I have felt it has got better as I am focussing on the nice parts of them getting older (e.g: watching them grow up and mature, and their personalities develop, etc). Someone I confided in gave me some really good advice, which was "take off the rose tinted specs". As much as there were the lovely parts of the younger years I'll miss, it is all too easy to romanticise how it was and forget how stressful it all was. It was more relentless, the kids were more dependant on me, the anxiety of being responsible for such vulnerable little ones was high, the house was a tip, I barely had time to be 'me' and look after myself (e.g. brush or wash my hair), DH and I spent much less time together without the kids, less freedom generally, and on the whole, life was more stressful (although I wouldn't have changed it).
These days I yearn for weren't all sweetness and light, and it is important to remember this when feeling melancholy. Also, it is the natural progression and how it was meant to be from the moment your children were born.
Starting primary school is definitely one of the largest emotional hurdles in bringing up children as it is the first huge milestone and comes immediately after the baby and child stage so you are bound to be feeling melancholy. However, if you feel it is taking over your life, maybe you could be a little depressed and it might be worth confiding in your gp. I know I have felt better as time has gone on so I am sure it will for you too 