Woodland. I am so sorry. That must be beyond excruciating.
I like the idea: "Always believe people are doing the best they can even when they aren't." This is a bit like qtpi pointed out in that it has helped me realize that the best my Qualifier could do was exactly what he did. He could not possibly have been the partner I wanted and needed. This helped me forgive him.
Also a monk once told me that the first step towards forgiveness is recognizing that you are not ready to forgive. It takes a long time to come to peace with the hurt that comes in a relationship with an addict and "time takes time."
Please go easy on yourself. Feel the pain and grieve the loss. It is so so hard. Let us know how you get on.