How do you cope with losing a child

[Mom]

I feel like there must be other moms out there, like me, who have lost their young daughters to breast cancer who might have suggestions for coping. We lost Tera in April (at the age of 36) after a 4 year battle. She was living the life most dream of before being diagnosed - great husband, beautiful home, young daughter (and looking forward to trying for more children), job she enjoyed, etc. There was no history of breast cancer in our family, but she was already getting mammograms due to Fibrocystic Breasts. Her tumor was at about 7 cm before it was discovered (by breast exam) and she'd had a mammogram a few months prior which didn't show it (although I am interested in having all of her scans and records reviewed to see if someone missed it).
In August I had a mammogram that revealed calcifications and ultimately had a double mastectomy 3 weeks ago due to DCIS.
As if the grief is not enough, I now deal with the guilt of why she had this horrible disease and died an agonizing death while my DCIS was caught before it ever became invasive. There was already the "normal" guilt that a mom has for not being able to protect a child, but now the "why her and not me". It's very hard to rejoice and be thankful that mine was just DCIS when she ended up with Stage IV that eventually went to her brain (among other organs).
When I am alone and my mind is idle it always goes to the darkest places and I can ONLY remember the really bad times and particularly the last 3 months of my daughter's life which included much hospital time as well as a hospice facility.
How do I cope? Has counseling been helpful for others? I still go to church, but I am so angry with God - I feel like I don't really know what to believe any more. How can God do this - or did he do this? He didn't answer any of my prayers.
If you are a parent who has lost a daughter to BC, do you have any words of encouragement? How can we possibly deal with this?