Look at the positives you have going. Your dad made it through a very tricky surgery. Positive one. (I'm sure he'll be fine, btw. You and your family will be in my thoughts!) Your son will still love you as much as ever-- he'll just do it at a velocity approaching the speed of light.
Positive two. (I truly wouldn't worry as much about the diagnosis. I work with children on a daily basis-- several of whom have been diagnosed ADHD. They're my favorite kids because they always keep me on my toes!) Not sure what kind of business your husband does, but call him up at a random moment-- hopefully when you know he won't be busy with business-y things
-- just to say that you miss him and love him. (Which I certainly hope you do!)
Remember to take a moment, or ten, for you and only you. Find a park for your son to play and you to sit and just enjoy the moment. Find a babysitter or a friend's house for your son when your husband gets home and plan a night just for the two of you. Make a silly card for your dad and take it to him in the hospital, just like I'm sure you probably did when you were a kid. I know it's hard to do, but try not to focus on the 'negatives', keep things on a positive.
Once all of that is brought into focus, I truly think you'll find time to write that you didn't even realize you had.
And remember-- you have a ton of friends here to vent to at any given moment of any given day!
Although I'd be wary of taking advice from a bunny tied up in a straight jacket freely admitting to being both cute and psycho...