Not the midlife crisis I was planning on...

And here's another 5 year survivor!  I had my only child at 36, elderly primagravida indeed! But she was 30 by the time I was diagnosed, and yes it makes a big difference. Your child is always your child but she was old enough to fully understand and to talk with. You can only think positive, the other option isn't worth giving house room to, because you have treatment options and time. Your capacity to articulate the issues and analyse how you are thinking will hold you in good stead. I found writing those feelings down useful.  You'll have rotten days. And you'll get over them. Attitude may not be everything but it makes this unwelcome journey just a bit easier. We all like to plan but as the old saying goes, life is what happens when you were making other plans. Your life has taken an unwelcome turn, but it will take other turns. 
And it's still your life, your story, still to be revealed. My best wishes.