Today would have been my due date...
In what sense would it have been your due date?
In the sense of being pregnant... I would have been a mother.
It seems that you lost your baby, perhaps through a miscarriage. You should know that it is normal and okay that you grieve the baby that you lost, especially as milestones pass.
You are right, even my doctor said it was a miracle though because I miscarried 11 or 12 weeks ago prior to that again.
Grieving the loss of babies is something that is difficult for anyone. Take all the time you need in order to recover. Not sure you can recover completely but with time it gets slightly easier.