I understand why you feel upset, having your partner there to support you in labour and meet their baby is how pretty much everyone imagines it, so your DH not being there , I can understand why it would rankle.
It is normal to regret this and feel sad about it.
However, you can't change it, so you need to find a way to accept this and move forward
I presume your DH is an involved father? That he is building a relationship with the baby and with you as a mother, and as a family of 3?
Agree with imfab, your DH did not miss the birth because he was in the pub or off his head, or with another woman, but because he was working....
the birth is part of the journey, and it is an important one BUT the next 18 years are also pretty important, you need to try to put this to rest, for your own peace of mind .... take time to be upset and be sad,but then move forward, enjoy wathcing your H with the baby and doing things together, the pain of him not being there will fade