ImperialBlether You're right, I need to make the most of now and just look back with happiness.
If I can very gently and with complete kindness, understanding and sympathy, give you all a gentle slap 
Please try and do what you have said above OP.
As the DD of a DM who poured her whole identity into being a mother I was made to feel that every stage of development I achieved was a slight against her. That I was somehow being disloyal, disobedient or ungrateful simply by growing up. My achievements felt mourned rather than celebrated as it was always viewed under the guise of what she was loosing, how her life was changing for the negative, how she was loosing her sense of purpose. It saddled me with a heck of a lot of guilt and negatively affected my life decisions.
My MIL is similar in that motherhood was her defining achievement in life and she is now very negative, jealous and bitter.
Your children did not ask to be born, they are merely on loan to you.
Please please please don't allow your sadness at times changing to influence how you communicate or interact with your DC. Celebrate them and rejoice in them growing up