I'm sorry SKW. I'm sorry you are going through this, that you're trying so hard and are so frustrated. I am sorry his words put so much on you. They DO tend to do that, shift the weight of the world onto our shoulders. (((hugs)))
Good for you for trying so hard, and for communicating with him. Good for him also for trying to be a good dad. Divorce is hard for everyone. It takes a while to find a new way for everyone to live. Sounds like he must have forgotten about the surgery and yes, that caused problems for you. I would expect this kind of thing to happen again.
I have learned that the more I try to explain myself to others, including explaining what difficulties they may cause me, the more I open myself up to them doing likewise to me. I honestly cannot handle other people's problems, it causes me too much stress. This has been a hard lesson for me to learn and has taken me MANY years to learn this about myself. It is a true limitation for me.
Your ex is likely overwhelmed and that's why he forgot. Alcoholics do not make good parents IMO, especially when they become divorced and there is no enabler there to do what needs to be done.