It's amazing, isn't it, how just the thought of not having to deal with a loved one's addiction on a daily basis can be so freeing?
DS was 3 going on 4 when I left AXH. He was 6 when I filed for divorce. After we left and before the divorce, we'd created a nice little rhythm for our lives that was only occasionally interrupted by the appearance of his father. The divorce, and the time leading up to it, created a huge upheaval. It was hard and so painful, probably for all of us, but I know it was hard on DS and I.
Even though it didn't feel like it (oh, man, did it ever NOT feel like it), going through the upheaval of a divorce is a relatively temporary situation. Eventually, it did settle down again. It may not have been exactly the routine or daily rhythm that I imagined, but it did stop feeling like a constant magnitude 8 or 9 earthquake.
Keeping a rhythm in place for DS for the times he was with me helped him adjust to the overall changes. It was something he could count on: our days will be about like *this* for the most part. I also found a play therapist for him and AXH's attorney FINALLY got him to agree to it part way through the divorce. She helped DS and gave me pointers on how to address issues when they came up.
I found that keeping DS and what was best for him in mind while I went through all of the divorce and custody things helped me not crumble completely.
Best wishes