[I'm 39, have daughters aged 5 and 7, and had a fist-sized brain tumor removed just over a year ago. So, though I'm doing fine right now (next MRI is tomorrow morning, hoping for another clean scan), I can certainly relate.]
If you have the strength, try to take up running. My world-class oncologist (at MGH in Boston) said he wished all of his patients were runners. Not occasional joggers, but real runners as in 3 to 5 miles multiple times per week.
Also avoiding refined sugars and red meat can't possibly hurt and there's a lot of evidence that it can make a huge difference.
More generally, I urge you to fight to stay alive. Anything is possible if you just hang on and keep alive. The pace of innovation and understanding in medical science is accelerating, and “incurable” diseases have been eradicated. HIV was once a death sentence; now patients live about as long as never-infected people. Just keep breathing, and hang on. Positive change is coming.
Sending all my positive vibes and loving best wishes your way.
I know you're inundated with messages from strangers but if you read this and want to talk or I can do anything for you, please do reach out. I'm "cweekly" on twitter and gmail.
PS This excerpt from Alan Watts -- "Think of Nothing" -- also helped me cope, when I thought my end was near. If you're a devout Christian you might find it offensive, but I (a humanist with zen buddhist leanings) found it comforting. http://www.youtube.com/watch?v=aQ5upMz0_ig&index=35&list=FLf...
And I intend to fight until I have nothing left. I refuse to just give up and die. It's not in my nature.