Hey there J.S., first of all: Keep your chin up! and Welcome to 3FC! Vent, question, comment, support and be supported, that's what we're here for.
So here's my 2 cents; this isn't a diet, it is a life change. If fast food is your vice... eat healthier fast food. There is Subway, Terriyaki is good, heck even McD makes salads and grilled chick. sandwichs.
Drink lots of water!!! I try to get 2 Liters each day. Diet soda is better than regular, but water is better than anything.
And for exercise, I know how painful knees really suck, just take it a little bit at a time. Remember that movement help the arthritis. And don't be afraid to try an 'elderly' program for awhile. Like physical theropy. If it really hurts that bad... anyhting is better than nothing. OH! water aerobics, i've heard, is wonderful.
hope I could help you a bit. I'll see you around, right?