I'm terminally ill and no one but you.
Why haven't you told anyone else?
If other people knew, I wouldn't be allowed to enjoy my life the way I'm used to.
That's an understandable fear to have and I really wish you weren't going through this. I hope you get better but you should still tell your family. Think about how hurt they'll be if they have to find out the hard way.
I'm not going to ruin their happiness. I'll take it to my grave.
What about after you die? They would be devastated to learn you kept this a secret the whole time. It would be much better if they learn about it and you can also tell them what you wouldn't want them to change.