((((northerer)))) Welcome to PC.
I am so very sorry for your loss. At the time you lost the preemies, did you go to counseling? grief support group? how did you and your ex-wife manage? It must have been terribly painful. Again, I am very sorry.
On the other hand, I am so happy for you that you found love, remarried and have kids and grandkids!
I can understand how you would feel sad, though
Many different circumstances in your life may be triggering very painful memories (trauma) from your past that were never completely resolved, so you are grieving again.
Have you spoken with your primary doctor and shared your concerns?
Also, would you consider going to a grief support group in your town and/or finding a T in your area that specializes in grief and loss?
Your feelings for your huge losses (and what could have been) certainly don't take anything away from your joy for your daughter and your grandchildren ... however, these circumstances may have brought back deep memories that are haunting you now that you have the opportunity to address so you can see things in a different light.
Please continue posting and let us know how you are doing.