At what point during weight loss do you hit a plateau?

Two months ago I weight 323 and for the past week I've been weighing 293 everymorning. Have I hit a plateau? I'm eating 2 cups of nonfat yogurt, 1 cup frosted mini wheats, 1 whole fruit (choice of apple, pear, orange), and 6 weight ounces of vegetable (choice of celery, carrot, tomato, lettuce, broccoli) each day, only drinking water (sometimes I add those crystal light packs to my water bottle, but I limit those to 2 per day), divided into 5 small "snacks." And thats all, do I need to amp something up or cut back on something? I'm also a dog groomer for a living, so I get excercise from that and I also walk 1/2 mile each evening before bed. What do I need to do so I can keep losing weight, I feel like I'm stuck!