I'm sorry for everything you are going through. This is such a hard process.
Honestly I would cancel. Therapy is a long term process, and I am sure you have been trying to get him to see stuff, acknowledge stuff, understand how you feel etc for months if not years... how could 1-2 therapy sessions that he didn't want to go to possibly get a better result? Normally the first few sessions are all just about letting people talk and getting to know the situation. It takes forever , if at all, to break through the defences and get people thinking differently. If your husband thought he owed you an apology, he would have apologised already, and if he really thought that maybe he would have taken some action to change. It is highly likely you will never ever get an apology. If you do, then it is much more likely to be in ten years, when the person is in a totally new chapter and can see things from a distance.
You are now separating, and while there is a big practical side to that, there is also a huge emotional side. It's very hard, but that means you must break away from trying to get him to understand, acknowledge, receive your rage... everything. That means not couples therapy, but your own individual therapy to help YOU with your mental health, to help you with your anger, so that you can start refocusing on your life, your grief, and your next chapter.
I know you want the separation to be amicable, and you are right that will be much easier. But individual therapy can also help with that. It can help you manage your emotions, validate your anger and release it in a way that honours it but is not destructive, help you know when to assert yourself and when to not sweat the small stuff, and help you rebuild yourself.
Shorter Couples therapy at this point, when you both have so much anger, sounds like it is just salt in the wounds.
By the way, it's extremely common when separating to paint the other person in a terrible light. It makes it more manageable psychologically, dulls the pain of loss and also the discomfort of personal responsibility. And anger is also a natural part of grieving. Your husband is doing all this and You are probably doing all these things also. It adds to the pain of the other person, but is a very natural part of the process.
You have every right to be angry, but at this point, I would try to go on a solo voyage to honour that anger and start to work through your grief. 