That's probably one of the most difficult situations for someone to be in. I really feel terrible for you. I know you're probably so confused right now. Ask yourself what you want right now. Do you want to stick with him through this? It won't be easy. I've watched a parent die from a terminal illness and it isn't easy. It's really hard to accept that the situation is beyond your control and there's nothing you can do.
But it can also be very rewarding. You can still be close to him and comfort him as much as possible.
Weigh the pros and cons of staying involved with him and then decide what you want from this. Do you think it's better for you to turn away and distance yourself? Or is that too hard for you to do? If you want to stay involved with him, talk to him. He's doing a very unselfish thing by trying to save you from as much hurt as possible. But you're a big girl and really you should be able to decide what you want to do here.
There isn't an easy way to deal with this. Not at all. I can give you some suggestions. Don't leave anything undone. Ask him what you want to know. Take pictures together. Record your favorite memories. Then he'll always be with you. You won't be able to forget and lose him. Most importantly, keep yourself healthy. It's easy to lose yourself when you're involved in something like this. Don't. Make sure you're eating and sleeping and exercising. Take care of yourself or you won't be of any use to anyone else.
This comes down to it being your decision, as hard as that is to hear. This is your life and you have to decide what to do with it. But hopefully I was able to help you out a bit. :)
You can talk to me anytime.
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And do adults just learn to play the most ridiculous, repulsive games?