I am so very sorry for your loss. I'm sure that all of the feelings you are having right now are very normal as they are a part of the grieving process.
But please don't beat yourself up with the "what if's" and the "if's" of all kinds. The truth is that you cannot control another person, you cannot change them, and you can't cure them. His addiction was HIS and only he could have fought that addiction.
Have you attended Alanon yet or spoke with a counselor or therapist? Or how about attending a support group for those that have lost a loved one - such as a widow's support group?
Please believe me, this wasn't your fault. Don't be so hard on yourself apart.