If I was you I wouldn't tell my mother yet. Just slack off with witness stuff little by little. And occasionally try to ask questions to get her to think. This will make her worry, but won't completely break her heart all of a sudden, and who knows you might help her start thinking.
I am really sorry you lost your father.
As I said before, use your last year in high school to do the best you can, save money, and talk to counsellors (there are way too many ways to spell that word and I don't know which is right and am unwilling to look it up!)
Definitely get involved socially in school this last year. Heck even try to go to prom if you can. Start living your life. But be very gentle with your mom.