I know exactly what you mean. My DH and I TTC for over 2 years, including a miscarriage in that time. We were exstatic when we found out we were pregnant again. I couldn't wait to nurse my baby, wear my baby everywhere. We were of course over joyed when we found out it was twins.
But it wasn't until after they were born that I had this sense of missing out. I'd find myself jealous of moms with one baby. I read on the LLL website that mothers of multiples often mourn that loss of a one-on-one mother-to-baby relationship that was soooo anticipated. When I read that, I thought, THAT'S IT!! that's this overwhelming grief I kept feeling. Don't get me wrong.. I LOVE my little guys. I feel blessed and lucky to have been chosen to raise and nurture these handsome little men in my life. However, they are my last, no more babies (tubal ligation during their c-section). I still feel grief and mourning for what I missed out on. And yes, I still feel jealousy when I see moms with their one baby. I wanted that again so bad.
So I see where you're coming from. That exclusiveness with one baby isn't there. And you feel bad. I do too.