I think I understand what you are feeling, if in a different way. You had your child, the one you wanted & were ready for, earlier than other people currently in your life. So your pregnancy, the birth of your baby, the milestones that your son has reached over the years, have already passed, & now you are watching others close to you experiencing them with their own family, for the first time.
My husband & I decided together we didn't want to concieve children... we wanted to adopt, but unfortunately, life circumstances got in the way of that also. I was OK with that... I -never- wanted to have a baby, instead we were going to adopt an older child (5-12 yrs). But when my sister got pregnant a few years ago, & I had a missed period, I actually got excited at the thought of being pregnant (after an initial total panic attack!). Turns out, I was not pregnant, simply very late... and the sadness I had at that moment was overwhelming. For something I didn't want to begin with.
Our emotions can play with us, based on the circumstances of what is going on around us, and even the time of life we are in. I think going forward, it will be important for you to acknowledge that you -did- experience what everyone else is going through right now.
Go back to the reasons you decided that you were done having children before. Are those reasons still valid? Are you & your husband on the same page about that decision?
If the answers to both are "yes", then validate that! Remind yourself that the choices you made were right for you, & the experiences you had are yours - they can't be taken away. Additionally, you will have new experiences to look forward to, both with your son & husband, as well as personal milestones & goals. Your son hasn't had his first GF, you & your husband haven't celebrated your 10yr Anniversary yet, etc.
For now, find a way to celebrate & take part in the events of your friends & family. Also, make sure to share what is going on in your life, so that they can share -your- family's events!
Wishing you the best in finding peace through these emotions... T.