Patty, definitely go with the sugar free. If you have to use full fat dairy products, just don't use as much. If you eat the sugar you will probably start to have cravings, and that will make it very hard to stay on plan. If you eat the fat, you won't crave, though your loss may be slower, but that is okay. And be careful about lowfat products in any case - sometimes the fat is replaced with sugar. The only lowfat/fat free stuff I buy is milk, yogurt, cheeses, and the reduced fat triscuits. Stay away from lowfat/FF salad dressings and mayonnaise.
Life's a journey, not a destination.
It's easier to stay on plan than to get back on plan.