I'm so sorry. I have to echo the suggestion to talk to your son (and I apologise if it is a bad suggestion for reasons I am unaware of). If he is 22 and independent, he is an adult who is old enough to know what it means that you have the kind of cancer you do and face it with you. It doesn't mean it won't be painful or that it would be easier to cope with, but it is generally more understandable to the average person in the context of terminal illness, and preparation might help him to come to terms with it and understand your frame of mind. The young and the healthy can sometimes have trouble imagining the extent of sickness and pain until it is tangible and they are forcibly confronted with it, whereas you have foresight about what is coming.
Of course, our emotions and instinctive reactions often belie what we can rationally process to be true. But I do think it might be better if you could talk about it, to make your wishes known, to give advice and talk about coping after your passing, and also to clarify your feelings. One of the hard parts of losing a parent is the perception, true or not, that they 'abandoned' you, or that they wouldn't have chosen to be with you - but there is a point one reaches, between the devil and the deep blue sea.
I do think it is ethically fine. Do your best by your child, but there is only so much in every human that they have to give, and a limit to how much good suffering can do anyone in the face of inevitable loss.