Hi. I have gone through a very similar situation with my husband, sveral times. He tried to leave, and we have been separated.
John's actions sound so much like Scott's. He wants to forget vs dealing with all of this. He most likely feels guilt and doesn't want to look at you as it reminds him of HIS guilt of putting you through this. Although you may not have ever said anything like this to him.
He believes that by leaving his life with you he will be leaving all the sadness and loss that cancer has brought upon him.
A new person represents the ability to leave all that crap behind. Not so. Unfortunatley he will eventually have to deal with all the pain of the cancer and he will grieve the loss of his marriage to you.
He can run for only so long.

I had to let my husband go and he almost lost everything. I too prayed every day.
He hit bottom and luckily I just happen to stop by his house and found him unconcious from not taking his morphine. He went into the hospital and then detox.
A new man arose. He is 53 days free from morphine and is making remarkable strdes.
All you can do is be there for yourself, your daugher and pray for him each day.
I will pray for you and John.