I think maybe it would help you to break it into two parts about drug use. There is the question on how the use of coke affects him. And then the second part is how his using coke and the likely change in behaviors affected you. My husband was snorting coke, but he cant control his use, I'll just say it has negative affects. I could see changes in him but at the time didnt know what it was from. On a face to face basis he was still ok, and didnt change his actions with me a lot. It reminded me of times when he legitimately had lot of stress at work and was preoccupied. When it got out of control, it became obvious and he had to get help in order to stop.
But this is your ex, so Im thinking his behavior made it that way? I think look back at how it was and honestly ask if they way things were was ok for you? Was his part of the relationship ok, how you were treated ok, think of the reasons why it ended. If those are still valid, then this might be enough to keep u moving forward?