How to prepare

When it comes to my cancer, I know that God and I have it beat. But still I can't help but be a little pessimistic sometimes. My doctor said that this cancer will eventually kill me. Whether it's a year from now, or 50.
God forbid it should be sooner, rather than later. The question comes up, how do I prepare my son for the day mommy might not be here anymore?
It's such a difficult question to not only ask but answer as well.
My heart breaks at the thought of not being around to see my son grow up. Or being able to grow old with my husband.
I guess the only thing I can do is keep trusting God that his plans are to prosper me and not harm me. I have to keep believing that with God, I've got this beat! Not just for a couple years, but forever.