Lise, your sitch certainly puts things in perspective. I don't know what I would do if my W decided (decides) to put some distance between us such that seeing my children, let alone sharing custody of them, would be a near impossible ordeal.
On how do you do it, I have a friend who asks herself that question all the time, and her STBXH is a substance abuser. She puts a lot of faith in God that the brief time her 18-month old visits with daddy that nothing bad happens. It's still always very painful and anxious for her, but I find her patience in the face of this quite remarkable.