you stated you had no knowledge of her doing this so you are not responsible for the loss of your son, imho.
as for grieving, i pick a quiet, private moment in my day and focus on my loss. i close my eyes. if other thoughts distract me i acknowlege them and then refocus on my loss. i allow the sadness to surface. i allow myself to feel it. i cry when i do this. i allow myself to cry and acknowlege my sorrow of the "what ifs". once i have cried as long as i need to, and sometimes it's a long time, i tell that someone how much i love them. i tell them i'm so sorry this happened. (the little one had no voice when she did this. it was not her choice to make, imho.) i tell them one day we will be together. i wish them well and tell them they will always have a piece of my heart.
sometimes i need to go thru the above more than once. it's ok to do it as often as is needed for you to find peace and acceptance of what will never be on this earth. then you may be able to heal from within. be kind to your "self".
i wish you peace in your heart.